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Nine years on: Thames Water's sewage works since the 2017 prosecution

J. Stanley Root · Oxford · working draft, 2 October 2026
Names of sewage works in blue open that works on the interactive map, in a second browser tab. Thames Water, Ofwat and the Environment Agency have not been asked to comment on this draft.

In March 2017 Thames Water was fined a record £19.75m for pollution from six sites. Ofwat later recorded that the case showed flow being held back from treatment so that the sampled effluent would pass. Nine years on, a capacity upgrade is reported finished at only one of the six. At least 31 of the 64 capacity upgrades the Environment Agency required across Thames's works by March 2025 missed their deadline, by a median of almost three years. Ofwat has fined Thames once for the failings and, Thames's own accounts record, opened a second investigation, into the delays. This report tells that story in seven short parts, for readers who care about a river, and introduces an interactive map that holds the evidence works by works.

Key points

1. How a sewage works is regulated and monitored

A works is allowed to spill only when it is already treating the flow its permit requires, and only because of rain. Everything in this report turns on that rule and on what was, and was not, measured to test it.

Three regulators share the job, and none owns the whole question. The Environment Agency writes each works' permit, inspects, and prosecutes breaches in the criminal courts. It also sets the legal programme of environmental improvements each company must deliver in each five-year period, the Water Industry National Environment Programme (WINEP). Ofwat sets the money each company may spend and recover from customers, and enforces the company's licence and its duty under section 94 of the Water Industry Act 1991 to keep its area "effectually drained". Government sets policy; the Urban Waste Water Treatment Regulations 1994 require works to perform under "normal local climatic conditions". [21] The Agency grades breaches under its Compliance Classification Scheme and sets the WINEP by legal "drivers". [22]

A permit carries two flow numbers. Dry weather flow (DWF) is the volume a works should receive on a dry day. Flow to full treatment (FFT) is the rate, in litres a second, that the works must treat fully before it may divert anything to its storm tanks; only when the tanks fill may it overflow to the river. [23]

The national design standard for FFT is three times the domestic and trade flows plus the maximum groundwater infiltration:

FFT = 3PG + Imax + 3E

P is the population served, G the flow per head, Imax the maximum infiltration and E the trade effluent. [23] The standard applies when a permit is written for a new or altered works. It is not retrospective: an old works keeps its old figure until someone changes it. [24]

A storm spill is lawful when the works is already passing forward at least its permitted FFT, the excess is caused by rain or snowmelt, it passes through the storm tanks first, and it leaves by a permitted, monitored outlet. It is unlawful when the works spills early (while treating less than its permitted flow), spills in dry weather, or spills through an unpermitted outlet. [25]

That produces two different gaps, with different owners.

Gap Between Lawful? Whose Remedy
1 the design standard and the permit Yes — no fine reaches it The permit-writer as much as the company Raise the permit and build the works to meet it
2 the permit and what the works actually treats No — a breach The company Operate, maintain and upgrade the works

A works permitted below the standard reaches its lawful spilling point sooner. On the register's recalculation, 109 of the 130 Thames works that can be tested are permitted to pass forward less than the standard requires. [26]

How compliance is tested. Since 2009–10 the companies have sampled their own treated effluent, usually once or twice a month on dates set a year ahead, and reported the results; the Agency audits the arrangements. [27] Whether a works was treating its permitted flow before it spilled was, at most works, not measured continuously until the 2020s. In March 2020 the Agency required certified flow monitors at works where compliance with the flow condition could not otherwise be confirmed. [28] Ofwat recorded that in 2020 none of Thames's 197 works with an FFT condition had a certified meter on that flow, and only one had a monitor on its spill to the storm tank. [1]

Spill monitors record duration, not volume. Event duration monitors record when an overflow spilled and for how long; every active overflow in England has had one since the end of 2023. The annual returns count spills on a "12/24" convention. [29][30] They record no volume and no quality.

Agency officers write a compliance assessment report after each inspection or data review, grading each breach from category 1 (major) to 4 (no impact) and recording the enforcement response. Inspections are targeted at known problems and are not a random sample. [22]

The point that matters for what follows. A works that cannot treat its load produces effluent that fails its samples. Let less sewage into treatment and the effluent passes, while the excess leaves untreated through the storm outlet. Set below what the permit requires, that is flow clipping. For most of the period, the test that was reported could be passed while the test that was not reported was failed. [3]

2. The 2017 case: six sites, and what the regime could not see

On 22 March 2017 at Aylesbury Crown Court, HHJ Francis Sheridan fined Thames £19.75m (cash) for pollution in 2012–14 from six sites; with costs of £611,140.06 the total was £20,361,140.06. The Environment Agency called it a record. [2]

Site Type Receiving water Fine (cash)
Aylesbury Treatment works River Thame £9.0m
Little Marlow Treatment works River Thames £8.0m
Henley Treatment works Fawley Court Stream £1.0m
Littlemore Pumping station (feeds Oxford works) — £0.8m
Didcot Treatment works Moor Ditch £0.8m
Arborfield Treatment works Barkham Brook £0.15m

What the Agency told the court. Untreated sewage had been diverted from treatment at "millions of litres per day" while "the incoming sewage flow was well within the designed capacity of the treatment works". [2] Its notice also records that management failed "to react adequately to thousands of high priority alarms". [31] Judge Sheridan was reported as saying: "It should not be cheaper to offend than to take appropriate precautions." [31]

What Ofwat later said the case showed. In its 2025 decision Ofwat records that the 2017 prosecution showed that at a number of works flow to full treatment "was routinely reduced and a portion of the flow was sent to storm tanks in order to ensure that the quality of the final effluent being discharged from the site met permit limits", a practice "known as 'flow clipping'". [3] It describes the failure to manage flow compliance identified in that prosecution as "a critical factor". [32]

Why the case is the regime in miniature. The quality of what left the works was sampled and reported. The flow let into treatment was not measured at most works. A works could therefore keep its sampled effluent within limits by sending more of its sewage to the storm tanks and the river. An independent analysis of Thames's fifteen-minute flow data by Windrush Against Sewage Pollution puts flow to treatment at Aylesbury in 2013 at about 80% of the permitted 715 litres a second in January and February and about 40% in March and April. [33] No court or regulator has ruled on that analysis.

What was and was not found about intent. The sentencing remarks are not published. In March 2017 the Bucks Herald reported Judge Sheridan as saying: "There is evidence of deliberate concealments of unlawful incidents." [34] That is a press report; the transcript, which can be ordered from Aylesbury Crown Court, should be read before it is relied on. No court or regulator has found that flow clipping was designed to conceal the condition of the works, and Ofwat found no deliberate concealment in its 2025 case. [16]

What Thames said. On the day of sentencing: "We deeply regret each of these incidents at six of our sites during the period 2012-14." It said the fine "will be paid in full by shareholders only", that it would add £1.5m to its community fund for the affected rivers, and that "in the three years since the last of those incidents we have learnt our lesson - there have been sweeping, far-reaching changes across the waste water business." [35][36][37] In 2018 it told Ofwat its own lesson: "Our company culture did not consistently value or promote the prevention of pollution incidents compared to other requirements such as sewage treatment works compliance, and health and safety." [38]

What followed inside the company. Thames set up an internal programme, "Go to Green", following the prosecution. [21] Its review of 2015–17 flow data found 106 of 250 works treating less than their permitted flow, 70 of them by at least 10%. [39] Ofwat found Thames "chose to prioritise action at fewer than 30" of the more than 100 works rated red or amber. [40] Part 5 sets out what Ofwat made of that.

3. The six sites, nine years on

Thames reports a capacity upgrade finished at one of the six; at two there is no capacity scheme in its plan at all. In March 2020 the Agency required new flow monitoring at all five treatment works and at the Oxford works Littlemore feeds, because compliance with the flow condition could not be confirmed. It required a flow increase at only two of the six sites, Henley and Oxford, and neither was delivered by its legal date. [28][41]

Site Permit as % of design standard 2020–25 legal obligation Capacity scheme in Thames's 2025–30 plan Web page, 2023 → April 2026
Aylesbury 78% Monitoring and investigation only None (sludge scheme only) 2028 → no date
Little Marlow 95% Monitoring and investigation only None (phosphorus only) 2027 → no date
Henley 74% at its 67 l/s permit Raise FFT 67 → 82 l/s by 31 Mar 2025 Reported finished 30 Apr 2026 2025 → 2026
Didcot 72% Monitoring and investigation only Growth scheme, forecast 31 Mar 2033 2028 → no date
Arborfield 91% Monitoring and investigation only Growth scheme, forecast 31 Mar 2031 2026 → no date
Littlemore → Oxford 88% at the new 1,434 l/s permit Raise FFT by 31 Mar 2024 Flow scheme, forecast 24 Dec 2029 "more than £130m", no date → early 2032

Sources: supporting register; Thames delivery plan data tables v3 (13 August 2026). Delivery-plan money in this part is Thames's forecast outturn as published; its price base is to be confirmed. [42][43][44][45][46]

Aylesbury (population equivalent about 131,500) is permitted to pass forward 715 litres a second against 921 required, 78% of the standard. It is on Thames's own list of works potentially non-compliant with their flow conditions, published by Ofwat. [43][47] Its web page gave a capacity upgrade "due to complete in 2028" from November 2023; in April 2026 Thames could not confirm the timing. The delivery plan's only scheme there is sludge treatment, £22.3m, forecast 2032. [4][48] In February 2026 inspectors found a rag-choked storm channel, a blocked scum trough and aeration equipment out of use, issued a site warning, and wrote: "There are multiple non-compliances on the site regarding general management and maintenance which should be rectified." [49]

Little Marlow (about 199,900) is permitted at 95% of the standard on the register's estimate. Its upgrade was "likely to complete in 2027" in May 2023; by April 2026 the timing was not confirmed, and the plan carries only a phosphorus scheme, forecast 2031. In 2024 it received 44,629.1 cubic metres a day in dry weather against a limit of 40,300; the Agency found "it is unlikely that the rainfall in 2024 was the only cause of the DWF exceedance". [44][5][50]

Henley (about 13,300) was prosecuted again in February 2021 and fined £2.3m for a pollution in April 2016. [51] Its legal obligation to raise treatment flow from 67 to 82 litres a second was due by 31 March 2025. In July 2025 the Agency recorded: "The site cannot meet the FFT of 82 l/s until the work on site is completed." In January 2026 inspectors measured 47.6 litres a second going to treatment. The plan reports the project finished on 30 April 2026 in its main table and 30 September 2026 in its appendix. [8][52][53]

Didcot (about 51,100) is permitted at 72% of the standard. Its upgrade was "programmed to be completed by 31/03/2025" in Thames's 2021 groundwater plan, "due to complete in 2028" on the May 2023 web page, undated in April 2026, and forecast in the delivery plan for 31 March 2033, with £0.57m of £87.6m spent. [45][6]

Arborfield (about 20,700) spilled in January 2025 while treating 127 litres a second against the 147 its permit requires. In April 2025 the Agency recorded: "TWUL staff informed the EOs that the capacity of the two storm pumps is not enough to pump storm sewage to the storm tanks effectively." Its web date went from 2026 to 2025, 2028, 2030 and then none; the plan's growth scheme is forecast for 31 March 2031. [54][55][56][7]

Oxford, which treats Littlemore's sewage (about 243,000), was due to raise its treatment flow by 31 March 2024. The Agency raised its permit in November 2025 from 1,040 to 1,434 litres a second before the works was built to meet it. Between 5 November and 31 December 2025 it discharged storm sewage for 237 hours while treating about 300 litres a second less than that, and the Agency opened an investigation. The plan forecasts the flow scheme for 24 December 2029. [9][57]

What the record does not show. It does not show what Thames has spent maintaining these sites since 2017: maintenance spending is not published works by works. [58]

4. The 2020–25 capacity programme, and where it stands in 2025–30

In March 2020 the Agency set Thames 64 legally binding actions to enlarge the capacity of 58 sewage works by March 2025. Thirty were to raise flow to full treatment and thirty-four to enlarge storm tanks; all 64 were graded certain to be required. [59]

The money. Ofwat set one allowance for each programme line in 2019 rather than approving schemes: £136.0m against Thames's request of £132.0m (2022-23 prices). In December 2024 it added £103.44m, so it has allowed £239m specifically for these upgrades. In August 2024 Thames forecast the programme at £524m — more than twice the allowance — of which £306m falls after the last legal deadline. [60][12][11] Thames's regulatory accounts record £216.5m spent on the two lines in 2020–25 against an allowance of £148.2m, both cash: 46% over. Across its environmental programme as a whole it spent 78% of its allowance, and on phosphorus removal 48%. [61][62] Money scheme by scheme was not published for 2020–25.

The dispute about scope. Thames's explanation is that the Agency applied "more challenge and expected more evidence" after the 2019 settlement, raising the required flow increase from 1,431 to 2,421 litres a second. Ofwat replied that, as it understood it, the guidance "was in place before PR19 FD and did not change" — but funded the larger scope in full. Both positions are on the record. [63][64]

The plan was back-loaded from the start. Thames's 2019 plan for storm tanks completed nothing in the first two years and left 78% of the storage to the final year. [65] Ofwat's 2025 decision found that in the early part of the period "the options to delay and defer spending on wastewater were actively considered by the Executive and Board". [66] The same decision saw "no evidence that the company sought to deliberately conceal the contraventions" and made no findings on permit breaches or criminality (Part 5). [16][17]

The delays. At least 31 of the 64 actions, at 27 works, were carried into 2025–30 unfinished. Every one is past both the Agency's deadline and Thames's own current regulatory date. The median delay is 2.95 years against the Agency's deadline; ten actions, at nine works, are more than four years late. [67][68] Thames's forecasts have kept moving: 26 of the 29 actions on its August 2024 list are now forecast later than it then said. [69]

Works Upgrade EA deadline Thames forecast (Aug 2026) Years late
Banbury Flow to full treatment 31 Mar 2023 29 Mar 2029 6.0
Fleet Flow to full treatment 31 Mar 2024 26 Mar 2030 6.0
Oxford Flow to full treatment 31 Mar 2024 24 Dec 2029 5.7
Maple Lodge Storm tanks 31 Mar 2025 29 Mar 2030 5.0
Moreton-in-Marsh Storm tanks 31 Mar 2025 4 Feb 2030 4.9
Slough Flow to full treatment 31 Mar 2025 18 Dec 2029 4.7
Leatherhead Flow to full treatment 31 Mar 2024 21 Aug 2028 4.4
Bourton-on-the-Water Storm tanks and FFT 31 Mar 2023 6 Aug 2027 4.4
Hogsmill Storm tanks 31 Mar 2024 25 May 2028 4.2
Fairford Flow to full treatment 31 Mar 2025 17 Feb 2028 2.9

The first nine rows are the ten longest delays (Bourton-on-the-Water has two actions); Fairford, on the Coln, is added for readers of Part 7. The full list of 31 is in the map's evidence layer "Capacity upgrade late". [70][20]

Reasons given. Thames gives a reason for delay in its tables for five of the 31 lines. At Cirencester it told inspectors in June 2025 that the wet weather of 2024 and the site's power supply had delayed the upgrade, which had been promised for March 2024. The inspectors wrote: "Delays to WINEP schemes are unacceptable – they will delay environmental improvement and risk failure of statutory requirements. Delays also risk failure to comply with permit conditions, which is a legal obligation." [71][72][73]

What Thames tells the public. Thames's page "Investment plans for storm discharge sites" promised quarterly updates in May 2023. Of the roughly 200 works for which it gave a completion date between May 2023 and May 2025, by April 2026 at least 75 were due later than first stated, by about 2.4 years on average, and 67 had lost their date and now say Thames is "not yet in a position to confirm the exact timing". No entry says its date has changed. [74][75][76][77] At nine works with overdue legal upgrades — Banbury, Fleet, Maple Lodge, Moreton-in-Marsh, Leatherhead, Hogsmill, Woking, Bampton and Fairford — the page gives no date while the delivery plan Thames filed for its regulator gives one. [78]

Year one of 2025–30. At 31 March 2026, 97 of the 98 carried-over schemes had a regulatory date on or before that day, and at least 284 of the 290 new wastewater schemes had recorded no spending. Part of the slow start is sector-wide: Ofwat's determination came three and a half months before the period began, and Thames deferred its appeal to the Competition and Markets Authority. That does not explain the carried-over schemes, 95 of whose dates had passed before the period began. [79][80] Of the new schemes' budgets, 46% is already forecast to fall after 2030. [79]

What Thames's accounts say. Its 2025/26 accounts state: "This means that new environmental permits have come into effect before the works necessary to achieve compliance with those new environmental permits have been carried out. Delayed WINEP 7/8 sites are at risk of operating outside the conditions of their environmental permits. Operating in breach of an environmental permit is a criminal offence." They add that the Agency "has not, to date, taken formal enforcement action. This is not to be taken as an indication that it will not take action." [81][82]

5. Ofwat's penalty of May 2025

On 28 May 2025 Ofwat fined Thames £104.5m (cash) — 9.0% of its 2023-24 wastewater turnover, against a statutory maximum of 10% — and imposed an enforcement order. [13][83] It found breaches of the 1994 Regulations, section 94 and Condition P of Thames's licence (adequate resources and systems of planning and control), "systemic across the company's asset base" and occurring "since at least 2017". [14][21] It could penalise only breaches after 8 March 2017, five years before its formal notice — a limit set by law, not by the evidence. [84]

The 157. Thames told Ofwat that 157 of its 236 works with flow permits or temporary consents were "sites of concern": 88 lacking the capacity to meet their permits, and 69 not operated or maintained to meet them consistently. The count is Thames's own, of works potentially non-compliant, not a count of breaches. [85][86][87]

What Ofwat found about decisions. It found that the Executive and Board "were made aware of the findings of the G2G risk assessment"; that Thames "chose to defer investment on addressing known compliance risks and focus resources on areas which, it acknowledges, were, historically, subject to greater regulatory scrutiny"; and that it "in effect, chose to accept ongoing non-compliance across a significant portion of its WWTW estate". [88][89][90] In August 2019 Thames had offered Ofwat a "lower cost, lower investment" scenario, which it said would reduce maintenance "across treatment works and pumping stations … to a level that does not keep pace with deterioration or the addition of new assets", with "an increased risk of breaching our discharge consents". [91]

What Ofwat did not find. In the same decision Ofwat saw "no evidence that the company sought to deliberately conceal the contraventions", found no evidence that Thames underspent its 2015–20 wastewater allowance or made financial gains, and made no findings on permit breaches or criminality: "We are not seeking to make findings about environmental permit failures or whether the acts of Thames Water or its employees, were criminal in nature." [16][17][92]

What the penalty is not. It is a share of one year's turnover, capped at the same figure however long the breaches ran. In Ofwat's words, "Our decision is not based on any conclusions as to the degree of harm caused to the environment or customers." [93]

What the order requires. By 28 November 2025 Thames had to agree a remediation plan with Ofwat and show it had reviewed every permit to check that its flow and storm-tank limits are adequate for the sewage received. [94][95] Neither the plan nor the result of the permit review has been published. [96]

The measure Ofwat used. The 157 measures works against their own permits. Of the 109 works permitted below the design standard, 32 do not appear on the list Ofwat published, including Banbury, permitted at 31% of what the formula requires. Their absence reflects the measure: a works that meets a permit set below the standard does not appear on a list of works failing their permits. [97][98]

Payment. Ofwat's fines of May 2025 totalled £122.7m; £18.2m arose, as reported, from a separate decision on dividends. As reported in August 2025, Ofwat agreed that 20% would be paid by 30 September 2025 and the rest deferred until shortly after recapitalisation, with 31 March 2030 as the backstop. [99][100]

6. Ofwat's second investigation: the delays

Thames's accounts record that Ofwat opened a separate investigation in February 2025 into the delays to the 2020–25 environmental programme. The 2025/26 accounts state that Ofwat issued it with a notice under section 203 of the Water Industry Act 1991 on 11 February 2025, "requesting the production of documents and information in relation to potential delays in TWUL's delivery of the AMP7 WINEP schemes by the original PR19 delivery date and the potential effects this may have on the delivery of the AMP8 WINEP schemes". [18]

What Ofwat is concerned about. In the accounts' words: "Ofwat is concerned that TWUL may have breached Condition P of its licence, in addition to Section 94 of the WIA91 and Regulations 4 and 5 of the Urban Waste Water Treatment (England & Wales) Regulations 1994." These are the provisions under which Ofwat penalised Thames in May 2025. Thames responded on 14 March 2025; Ofwat issued second and third notices in January and February 2026, and Thames responded in February and March 2026. [18]

What it could lead to. The accounts record that Ofwat's powers include "the potential imposition of an enforcement order, the acceptance of enforceable undertakings and/or the imposition of a financial penalty on the Company of up to 10% of annual turnover of the relevant regulated business". No outcome has been found in the public record. [18]

What the Environment Agency has recorded. Its environmental performance assessment for 2024 rated Thames one star, a "poor performing company", and recorded 75.2% delivery of its environmental programme in the year to March 2025, with 121 schemes missing their deadline. [101] Its inspectors have called delays "unacceptable" at Cirencester, Faringdon and Fairford (Parts 4 and 7). [73][102][103] Thames's accounts state that the Agency has not, to date, taken formal enforcement action on the delayed schemes. [82]

Why it matters now. The company's senior creditors, seeking control as London & Valley Water, have proposed that the company "would not be subject to the outcome delivery incentives regime during AMP8" — the rewards and penalties for performance in 2025–30 — while committing to pay outstanding fines. [104] On 18 September 2026 the Commons Environment, Food and Rural Affairs Committee urged the government to reject relief from fines, saying it "will be unacceptable to the public who will see it as rewarding failure". [105] A revised offer was reported on 23 September 2026; whether the March terms still stand has not been established. [106]

7. The map: the evidence, works by works

The report is the story; the map holds the evidence. Thames's sewage treatment works are on it — 348 permits at 346 works — each placed at its outfall, with its receiving water and the river system it drains to. The evidence in the map's popups is read by script from the supporting register, joined on the Environment Agency permit number, and carries a note of the sheet it comes from; the 2017 fines are taken from the Environment Agency's notice of that year. [107]

How to use it.

  1. Start with a river. Tick a river system — Cherwell, Thame, Colne, Kennet and so on, or "Select all" — and its works appear, labelled, sized by population equivalent. The Cotswold Coln has its own entry, "Coln (Cotswolds)", kept apart from the Hertfordshire "Colne (Herts)"; ticking either shades its catchment and picks out its river.
  2. Or start with a size. The size buttons (100k+, 50–100k, 10–50k, 2–10k, under 2k, All) show one band or several, alone or combined with a river; "None" leaves size out.
  3. Ask a question of the evidence. Tick an evidence layer and only the works with that evidence show, each with its key figure in red above its name — its percentage of the standard, its fine, its delay in months. Tick two and switch between "any" and "all": for example, works on the Ofwat list and with a late upgrade.
  4. Read a works. Click any dot or red name. The card sets out, in the same order for every works: capacity against the design standard, five years of spills, whether it is a site of concern with or without a capacity scheme, any late upgrade, Thames's promises against its latest plan, and what the Environment Agency found, in its inspectors' words.
  5. Follow this report. In the web version, every works named in blue opens that works on the map.

The evidence layers. Counts are distinct works in the register.

Layer Works What it shows Source
Permit below design standard 109 Permitted FFT below 3PG + Imax + 3E EA permits, tested against the design standard
… below 80% of the standard 46 The same, below 80% EA permits, tested against the design standard
Below required capacity (extended / ORIC test) 174 Includes works the register cannot test, on the Oxford Rivers Improvement Campaign's inputs Ofwat ORIC inputs
The six sites of 2017 6 The Aylesbury case Court record 2017; Thames delivery plan
Court fines (local press, 2010–26) 19 Fines as reported in local papers Local press reports
On Thames's own list of potentially non-compliant works 155 Ofwat decision, App. A3, as held in the register (157 sites, 155 matched to a works) Ofwat final determination, App. A3
Ofwat site of concern, no capacity scheme in the 2025–30 plan 76 App. A4 sites against the delivery plan Delays workbook
EA inspection recorded a breach 178 Oct 2024 – Jul 2026 EA compliance assessment reports
… a Category 1–2 breach 67 Major or significant EA compliance assessment reports
… investigation or enforcement being considered 71 As recorded on the report EA compliance assessment reports
Flow clipping recorded by the EA 2 Reading and Camberley EA compliance assessment reports
Capacity upgrade late 27 The 31 late actions EA WINEP; Thames delivery plan
AMP7 schemes carried into AMP8 83 Any carried-over scheme Thames delivery plan, App. A
Web page: date withdrawn 67 "Not yet in a position to confirm the exact timing" Thames website pages
Spilled 1,000 hours or more in 2025 23 EDM annual return EDM annual return

Two rivers, read through the map.

The Coln. Open the Coln on the map → Four Thames works discharge to the Coln: Andoversford, Withington, Bibury and Fairford. [108] Fairford spilled for 4,332 hours in 2024, 49% of the year. [109][110] Its legal obligation to raise its flow to the design standard was due by 31 March 2025. The Agency raised its permit to 56.8 litres a second on 17 June 2025; the works was not built to meet it. [111][112] In December 2025, after a member of the public reported sewage entering the Coln, officers found the works "discharging to and from storm whilst not meeting its permitted full flow to treatment (FFT)" and wrote: "Delays to schemes are unacceptable … Delays have led to permit condition breaches." They issued a site warning. [103][113] Thames now forecasts February 2028; its web page gives no date. [19][114] Upstream, the Agency recorded Bibury's dry weather flow limit exceeded in each of the nine years to 2024 and called the delay "not acceptable". [115] Andoversford's permit lets it divert to storm at 2.1 litres a second, below its own permitted dry weather flow; it spilled for 2,476 hours in 2024, though its dry-day flows are within its permit and Thames says it regularly treats more. [116][117][118]

The Colne. Open the Colne on the map → Thirteen Thames works lie in the Environment Agency's Colne catchment, which takes in the Ver, Gade, Chess and Misbourne. The largest by far is Maple Lodge (population equivalent 595,844), permitted at 81% of the design standard. [119] Its storm-tank upgrade was a legal obligation due by 31 March 2025; Thames forecast August 2030 in August 2024 and now forecasts 29 March 2030. Its phosphorus scheme, legally due by 22 December 2024, is forecast for the same date, at £105.89m with £5.64m spent (delivery plan, as published). [20][120] Its web page first gave 2030 in late 2024 and now says the timing cannot be confirmed. [121] In July 2025 the Agency recorded that Maple Lodge's "flow records were classed as “suspect” or “missing” for a total of 259 days" in 2024, a category 2 breach, and the report ended in an investigation. [122]

Cautions. Inspections are targeted, so their counts describe what the Agency recorded where it looked, not the condition of every works. Spill hours measure duration, not volume. A permit below the design standard is lawful; it was written by the Agency. Press items are what papers reported, not findings. Delivery-plan money is as published, on Thames's output-only basis; its price base is to be confirmed, and it is never added across lines. Name-matching between Ofwat's lists and the register is approximate.

Where it is. The map is a single web page that opens in any browser. It is a private working draft, and will be checked again before it is shared more widely.

What this report does not establish

It does not establish that any person acted with intent to deceive or to cause harm. Where a court or regulator has used strong words they are quoted and attributed, and where Ofwat found no deliberate concealment that is stated alongside. It does not establish the condition of any individual asset, or which works are now operating outside their permits. It rests largely on Thames's own published figures; if those are wrong, so is what is built on them. Thames Water, Ofwat and the Environment Agency have not been asked to comment on this draft.

Sources

Each number in the text refers to the source below. “Supporting register” is the workbook of 2 October 2026 from which the counts and calculations are made; most entries are taken from the author’s full briefing (version 13, 2 October 2026).

  1. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.15 fn 67, p.51.
  2. Environment Agency, “Thames Water ordered to pay record £20 million for river pollution”, press notice, 22 March 2017 — https://www.gov.uk/government/news/thames-water-ordered-to-pay-record-20-million-for-river-pollution (text read from the Wired-Gov copy).
  3. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.22, p.55.
  4. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 18 May 2024 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  5. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  6. Thames Water, Didcot Groundwater Impacted System Management Plan, original version, October 2021. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  7. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme. Thames Water, AMP8 Delivery Plan data tables v3, 13 August 2026, table DPWW4, and the earlier published version held in the supporting register; see also the supporting register.
  8. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  9. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  10. Supporting workbook on the 2020–25 capacity programme.
  11. TMS-DD-057, p.12, Table 7 (2022-23 prices).
  12. Ofwat, PR24 final determination model PR24-FD-CA103-PR19-WINEP-carryover.xlsx (December 2024), sheet “Deep dive TMS WINEP waste”, cell B14, section (i).
  13. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), executive summary (also paras 1.3 and 6.58), p.6.
  14. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), executive summary, p.3.
  15. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.48, p.177; para 4.97, p.158.
  16. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.50, p.178.
  17. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), executive summary, p.6.
  18. Thames Water, Annual Report and Financial Statements 2025/26, note 26 “Contingent liabilities”, item 4, p.154.
  19. Supporting register; Thames TMS-DD-057 Annex 1, August 2024; Thames AMP8 Delivery Plan data tables v3, 13 August 2026, DPWW4 row 62 and Appendix A.
  20. Supporting register: Environment Agency WINEP3 (March 2020) deadline; Thames AMP8 Delivery Plan data tables v3 (13 August 2026), DPWW4 columns L and M; TMS-DD-057 Annex 1 (August 2024); Appendix A (August 2026).
  21. Ofwat, Notice of Ofwat’s decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025, redacted, 237 pp (PDF). Passages quoted: executive summary (obligations breached; five causes; “systemic”; “since at least 2017”); paras 2.37–2.41 and footnote 15 (FFT, 3×DWF, causes of FFT failure, catchment change); paras 3.13–3.19, footnotes 66–70 and table 3.1 (354 works; 197 FFT permits; 39 Temporary Deemed Consents; 2020 monitoring; the 92% method; 104, 137 and 157 sites; 88 capacity and 69 operation or maintenance); paras 3.21–3.22 and footnote 73 (2017 prosecution, flow clipping, end-of-pipe focus); paras 3.91–3.95 (SOAF reasons; 52 of 84); paras 3.35–3.36, 4.63–4.66 and 4.97–4.100 (Go to Green; funding; “chose to accept ongoing non-compliance”; Condition P from March 2017); para 6.4 and 6.18 (penalty window from 8 March 2017); Appendix A1, Requirement 1(a), (c), (d) and (e) (remediation plan, root cause analysis, triggers, annual catchment review, permit review); Appendix A2 paras 16–17 and 75 (Thames’s responses); Appendix A3 (list of 137 potentially non-compliant sites on 2021 data).
  22. Environment Agency, Compliance Classification Scheme guidance; Environment Agency, Water Industry National Environment Programme (WINEP3) national dataset, March 2020, driver definitions.
  23. Environment Agency, “Water companies: environmental permits for storm overflows and emergency overflows”, 13 September 2018 (FFT = 3PG + Imax + 3E) — https://www.gov.uk/government/publications/water-companies-environmental-permits-for-storm-overflows-and-emergency-overflows.
  24. Lord Crickhowell, chairman-designate of the National Rivers Authority, House of Lords, Water Bill, 17 April 1989 (Hansard): “The truth is that at present we have a system of discharge consents that is to a significant extent not based on objective standards but designed to do little more than maintain the status quo and ensure that at least the situation does not get worse.” Environment Agency, Water companies: environmental permits for storm overflows and emergency overflows (guidance): the flow-to-full-treatment formula applies when the Agency “issues permits for all new, improved or altered storm overflows”. The Storm Overflows Evidence Project (2022) records that “many permits are historical” and that the cost of revising them is significant. Standard permit wording quoted from an Environment Agency permit for a comparable works (Colchester, ASETS/1046, 2018); the same guidance states: “We’ll normally refuse permits for overflows to relieve surcharge due to groundwater infiltration. You must prevent leaks in accordance with BTKNEEC.”
  25. Ofwat, Notice of decision, 28 May 2025, footnote 5: “The Environment Agency currently defines dry day spills as spills occurring on any day when there has been less than 0.25mm of rainfall in the upstream catchment on that day and the preceding 24 hours.”
  26. Supporting register (26 September 2026), count of works permitted below the design standard.
  27. Defra, “Coverage of operator self-monitoring”, 8 March 2023 — https://deframedia.blog.gov.uk/2023/03/08/coverage-of-operator-self-monitoring; Environment Agency, “Water companies: operator self monitoring (OSM) environmental permits”, updated 17 March 2026 — https://www.gov.uk/government/publications/water-companies-operator-self-monitoring-osm-environmental-permits.
  28. Environment Agency, Water Industry National Environment Programme (WINEP3), national dataset, issued 27 March 2020: Thames actions and driver definitions U_IMP5, U_IMP6, U_MON3–5; transcribed in the supporting register.
  29. Defra, Storm overflows monitoring hits 100% target (notice): 7% of overflows monitored in 2010; instruction to increase monitoring in 2013; all overflows monitored by the end of 2023. Defra press office, 1 April 2021 (blog): first publication of annual returns, for 2020, on 31 March 2021. Environment Act 2021, sections 80 and 81 (legislation.gov.uk).
  30. Environment Agency, Event Duration Monitoring storm overflow annual returns, 2021–2025, and guidance on the 12/24 counting method.
  31. Environment Agency press notices for each case, including: record £20.36m fine, 22 March 2017 (copy); Princes Risborough, March 2016 (copy); Idbury, [2019] EWCA Crim 1344 (judgment); Maidenhead, 10 July 2019 (notice); Chislehurst civil sanction, 22 January 2019 (notice); Henley, 26 February 2021 (notice); Hogsmill, 26 May 2021 (notice); Oxford, 19 November 2021 (notice); Enborne, [2015] EWCA Crim 960; Camberley, February 2015; Crawley, July 2023. Parliamentary written answer HL843 (2020) lists twelve Environment Agency prosecutions of Thames Water from 2015 to 2019 (answer). Judge Sheridan’s remark as reported by Kingsley Napley and the Daily Mail, 23 March 2017. Open investigations: Insurance Journal, 22 May 2025.
  32. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.49, p.178.
  33. Oxford Rivers Improvement Campaign, Thames sewage works capacity workbook, sheet “Required capacities”, as reconciled in the supporting register (26 September 2026). P. Hammond, Windrush Against Sewage Pollution, review of Thames Water sewage works, Part 1, Aylesbury STW section (Environment Agency storm-overflow table for Thames, 20 May 2020: Aylesbury permitted pass-forward flow 715 l/s; fifteen-minute FFT data for 2011–13). Environment Agency, EDM Storm Overflow Annual Returns 2021–2025, all water and sewerage companies, and summary data tables; supporting register. Environment Agency, Consented Discharges to Controlled Waters with Conditions, August 2026 extract: live final-effluent permits, dry weather flow and flow to full treatment.
  34. Bucks Herald, “COURT REPORT: ‘A Failure waiting to happen’ – Thames Water Utilities fined £20m …”, 22 March 2017 — https://www.bucksherald.co.uk/news/court-report-a-failure-waiting-to-happen-thames-water-utilities-fined-aps20m-for-pumping-15-billion-tonnes-of-untreated-sewage-into-river-thames-731557 — reporting Judge Sheridan’s sentencing remarks (unpublished): “There is evidence of deliberate concealments of unlawful incidents.” Quotation checked word for word against the transcribed text of the article on 2 October 2026. Press report only; confirm against the Aylesbury Crown Court transcript.
  35. Bucks Herald, “Thames Water: ‘We deeply regret’ pumping sewage into waterways”, 22 March 2017 (https://www.bucksherald.co.uk/news/thames-water-we-deeply-regret-pumping-sewage-into-waterways-735992), Thames Water statement.
  36. Bucks Herald, 22 March 2017 (as above), and Bucks Herald, “COURT REPORT: ‘A Failure waiting to happen’”, 22 March 2017 (https://www.bucksherald.co.uk/news/court-report-a-failure-waiting-to-happen-thames-water-utilities-fined-aps20m-for-pumping-15-billion-tonnes-of-untreated-sewage-into-river-thames-731557), Thames Water statements.
  37. Oxford Mail, “Thames Water fined £20.3m for polluting Thames”, 22 March 2017 (https://www.oxfordmail.co.uk/news/15173465.thames-water-fined-record-20-3m-polluting-thames-1-9-billion-litres-untreated-raw-sewage/), Thames Water statement.
  38. Thames Water, PR19 business plan, Appendix 1 “Past performance and lessons learnt” (September 2018), p.8 (Internet Archive copy); transcribed in the supporting register.
  39. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.32, p.58.
  40. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 4.62 (also 3.34), p.148.
  41. Environment Agency, Water Industry National Environment Programme (WINEP3), national dataset, issued 27 March 2020: Thames actions and driver definitions U_IMP5, U_IMP6, U_MON3–5; transcribed in the supporting register. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entries as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme;.
  42. Thames Water, AMP8 Delivery Plan data tables v3, 13 August 2026, table DPWW4, and the earlier published version held in the supporting register; see also the supporting register.
  43. Oxford Rivers Improvement Campaign, Thames sewage works capacity workbook, sheet “Required capacities”, as reconciled in the supporting register (26 September 2026).
  44. Oxford Rivers Improvement Campaign, Thames sewage works capacity workbook, sheet “Required capacities”, as reconciled in the supporting register (26 September 2026).
  45. Oxford Rivers Improvement Campaign, Thames sewage works capacity workbook, sheet “Required capacities”, as reconciled in the supporting register (26 September 2026).
  46. Supporting register; percentages for Henley, Arborfield and Oxford computed from the permitted and required flows.
  47. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), Appendix A3, p.210.
  48. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 18 May 2024 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme. Thames Water, AMP8 Delivery Plan data tables v3, 13 August 2026, table DPWW4, and the earlier published version held in the supporting register; see also the supporting register.
  49. Environment Agency compliance assessment report C0006832, Aylesbury STW, inspection 16 February 2026 (supporting register).
  50. Environment Agency compliance assessment report R/581717; transcribed in the supporting register.
  51. Environment Agency press notice, Henley, 26 February 2021.
  52. Environment Agency compliance assessment report S/566776, Henley STW, inspection 2 July 2025 (supporting register).
  53. Environment Agency compliance assessment report C0006724, Henley STW, inspection 28 January 2026 (supporting register).
  54. Environment Agency compliance assessment report I/545751, Arborfield STW, incident of 23–24 January 2025, assessed 7 March 2025 (supporting register).
  55. Environment Agency compliance assessment report S/555620, Arborfield STW, inspection 29 April 2025 (supporting register).
  56. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  57. Environment Agency compliance assessment report C0012484 (permit CTCR.0709), assessment 23 February 2026; transcribed in the supporting register.
  58. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), entry as captured by the Wayback Machine on 18 May 2024 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme. Thames Water, ‘Investment plans for storm discharge sites’, entries as captured by the Wayback Machine on 25 May 2023 and as live on 26 September 2026; supporting workbook on the 2020–25 capacity programme.
  59. Environment Agency, Water Industry National Environment Programme, WINEP3 national dataset (March 2020, public version), sheet WINEP DATA: Thames rows (column I), primary driver U_IMP5 or U_IMP6 (column S), completion date (column Z), certainty (column AA); counts in the supporting workbook on the 2020–25 capacity programme.1, 1.U_IMP5 and 1.U_IMP6.
  60. Thames Water, TMS-DD-057 “WINEP7 Carryover” (response to Ofwat’s PR24 draft determination, August 2024, redacted), p.4, Table 2 (2022-23 prices); the same figures in 2017-18 prices are in Ofwat’s PR19 final determination feeder models FM_E_WWW_flow-to-full-treatment_FD.xlsx and FM_E_WWW_storm-tank-capacity_FD.xlsx (December 2019), sheet Allowance, row TMS, columns D (request) and G (model), converted at Ofwat’s factor of 1.18063; scope from.
  61. Thames Water, Annual Performance Report 2024-25, data table 4M (Ofwat template v1.1), rows 21 (line 4M.12, flow to full treatment) and 24 (line 4M.15, storm-tank capacity), columns AB (cumulative expenditure on all schemes, AMP7 only) and AC (cumulative allowed expenditure); cash. Ofwat restatement: PR24-FD-CA96-PR19-Expenditure-model.xlsx (19 December 2024), sheet “Wastewater spend -excl CWW12&17”, cells K8, K9, K144 and K145.
  62. APR 2024-25, table 4M, rows 46 (line 4M.37, phosphorus removal) and 56 (environmental programme total), columns AB and AC; cash.
  63. Thames Water, document TMS-DD-057, §4.1, p.8, and §5.2. Ofwat, PR24 final determinations, PR19 WINEP carryover workbook (reference PR24-FD-CA103), sheet ‘Deep dive TMS WINEP waste’, cell B14.
  64. PR24-FD-CA103-PR19-WINEP-carryover.xlsx, sheet “Deep dive TMS WINEP waste”, cell B14, section (i). Ofwat’s £23.06m and £80.38m are Thames’s own figures: TMS-DD-057, pp.8–9.
  65. Supporting workbook on the 2020–25 capacity programme.STM.plan.2020-21 to 2.STM.lastyr_share_m3.
  66. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.22, p.171.
  67. Thames Water, AMP8 Delivery Plan data tables, version 3 (13 August 2026), worksheet DPWW4, and Appendix A “WINEP completion dates” (August 2026); Thames Water, Delivery Plan 2025/26 Full Year Report (August 2026), p.26 (definition of interim milestone 6). Matched to WINEP3 by WINEP identifier: supporting workbook on the 2020–25 capacity programme.
  68. Supporting workbook on the 2020–25 capacity programme: WINEP3 column Z and DPWW4 column M to the Appendix A date for each action; years are days divided by 365.25. Using DPWW4 column AB instead of Appendix A for the seven actions where the two differ leaves both medians unchanged.
  69. TMS-DD-057, Annex 1, Table 22, against Appendix A (or DPWW4 column AB where Appendix A gives no date); supporting workbook on the 2020–25 capacity programme.cap.fc_vs_aug24 and 6.cap.fc_later_than_aug24.
  70. Supporting workbook on the 2020–25 capacity programme; supporting register.
  71. DPWW4, column AE “reason for delay”, as transcribed in the supporting register.
  72. Environment Agency, compliance assessment report S/565491, Cirencester STW, assessment 16 June 2025; supporting register.
  73. Environment Agency, compliance assessment report S/565491, Cirencester STW, assessment 16 June 2025; supporting register.
  74. Thames Water, ‘Investment plans for storm discharge sites’ (thameswater.co.uk > River health > Information about specific sites), as captured by the Wayback Machine on 25 May 2023, introduction, para 3.
  75. Supporting register (26 September 2026).
  76. Supporting register (26 September 2026).
  77. Thames Water, ‘Investment plans for storm discharge sites’, Wayback Machine capture of 25 May 2023 (20230525162648) and live page saved 26 September 2026 (20260926092537); automated text search of the live page for delay/revis/postpon/originally/moved/rescheduled/‘later than’: no hits.
  78. Supporting workbook on the 2020–25 capacity programme.
  79. Thames Water, Delivery Plan 2025/26 Full Year Report, August 2026, and the accompanying AMP8 Delivery Plan data tables, table DPWW4 ‘Scheme-level data – Wastewater’, position at 31 March 2026. AMP7 carryover: the Year 1 Baseline of March 2025 contained 124 outputs, reduced to 123 after the Banbury P-Green output (WINEP THM00078) was removed; 21 outputs were achieved in year one (17%) and 102 remain (83%); 29 slipped (24%), of which 19 by more than 90 days; total AMP7 carryover project cost £3,780m with £793m spent to March 2026, for 108 projects. AMP8 year one: actual enhancement expenditure £256.953m against a November 2025 baseline of £352.349m, with wastewater £133.478m against £213.819m. The published AMP8-to-AMP9 roll-forward baseline is £1,437.123m. Table DPWW4 as totalled in the supporting register: 388 wastewater schemes, of which 98 AMP7 carryover (budget £1,772.355m, spent £286.688m, 16.2%) and 290 AMP8 (budget £3,116.302m, spent £9.956m, 0.3%); 97 of the 98 carryover schemes carry a current regulatory date that had passed by 31 March 2026 (one in 2023, 69 in 2024, 26 in 2025); 284 of 290 AMP8 schemes at nil spend; £152.8m and £1,425.8m respectively forecast to fall into AMP9. Thirteen schemes have a formally revised regulatory date, slipping by a median of 12 months and a maximum of 24. The progress report and the data table give incompatible totals for the carryover programme; see B8.6.
  80. Ofwat PR24 final determinations published 19 December 2024. Thames Water, CMA deferral, October 2025 (statement): referral requested 14 February 2025, deferred from 18 March 2025 and on 21 October 2025 without an end date. Water Magazine, 5 June 2026 (article): “AMP8 has not truly started yet… What should have been year one has effectively become year zero.”
  81. Thames Water, Annual Report and Financial Statements 2025/26, notes to the consolidated financial statements, note 26 “Contingent liabilities”, item 3, p.154.
  82. Thames Water, Annual Report and Financial Statements 2025/26, note 26, item 3, p.154.
  83. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.3, fn 275, p.167.
  84. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.4 and fn 276, p.168.
  85. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.17, p.53.
  86. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.19 (Table 3.1), p.54.
  87. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.17, p.53; paras 6.42–6.58, pp.176–180.
  88. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 3.172, p.109.
  89. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.48, p.177.
  90. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 4.97, p.158.
  91. Thames Water, “Response to Ofwat’s PR19 Draft Determination”, TW-DD-001, August 2019, pp.6–7, 14, 102 and 104. Recovered from the Internet Archive.
  92. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.22, p.171.
  93. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), para 6.20 (repeated 6.29), p.171.
  94. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), Appendix A1, Requirement 1(a), p.182.
  95. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), Appendix A1, Requirement 1(e), p.183.
  96. Ofwat, Notice of decision to issue an enforcement order and impose a financial penalty on Thames Water, 28 May 2025 (redacted), paras 3.188–3.194 and fn 203, p.113 onwards; para 6.24, p.172; Appendix A1, Requirement 1(a), p.182.
  97. Supporting register (19 September 2026), computed 21 September 2026: 130 works with both a permitted flow to full treatment and a computable design requirement, (3PG + Imax + 3E) ÷ 86.4 l/s, with Imax estimated as the 95th percentile of dry-day flow less domestic flow; 109 permitted below it. Matched to Appendix A3 of the Ofwat decision by works name (column “Ofwat A3”). Spill hours are the works-located overflows in the Environment Agency event duration monitoring annual returns, 2021–2025, averaged per works and summed. Unit cost from the Cirencester upgrade (B3.6): £29m for an increase from 269 to 484 l/s, £134,900 per l/s. Drainage plan comparison from note 47 (175.24 Ml/d, 2,028 l/s, 26 schemes, 2025–2050). Off-list shortfall by works: Banbury 1,098 l/s, Beddington 989, Slough 579, Rye Meads 401, Didcot 128, Cirencester 126, Ripley 124, Wantage 101, others 388.
  98. Supporting register: 130 works tested, 109 below the design standard, of which 77 on Appendix A3 and 32 not; recounted for this briefing.
  99. Ofwat, penalty payment arrangement, 27 August 2025. The £18.2m penalty arose, as reported, from Ofwat’s separate decision of May 2025 on dividends, which has not been re-read for this briefing.
  100. Water Industry Act 1991, section 22A(7), (9) and (11) (legislation.gov.uk). Payment plan: Water Magazine, 27 August 2025 (article); Hargreaves Lansdown news, same date (article); Thames Water, Ofwat investigations payment plan, 27 August 2025 (statement): “These penalties will not be funded by customer bills.” Thames Water, six-monthly report under the enforcement order, November 2025 (PDF). Payment of the first instalment has not been confirmed from a published source. Ofwat’s own pages were not accessible for this check; its statements are as reported. On the criminal alternative: Sentencing Council, Environmental offences definitive guideline for organisations, which requires the court to ensure that the combination of financial orders removes any economic benefit derived from the offending.
  101. Environment Agency, Water and sewerage companies in England: environmental performance report for 2024 (October 2025), Thames Water figures as transcribed in the supporting register.
  102. Environment Agency, compliance assessment report S/563193, Faringdon STW, assessment 18 June 2025; supporting register.
  103. Environment Agency compliance assessment report C0004658, Fairford STW, third-party reported pollution incident, assessment 30 December 2025.
  104. Thames Water, “Recapitalisation update — press speculation on London & Valley Water’s proposal”, 16 March 2026 — https://www.thameswater.co.uk/news/2026/mar/recapitalisation-update-press-speculation-on-london-valley-water-s-proposal.
  105. House of Commons Environment, Food and Rural Affairs Committee, news release, 18 September 2026 — https://committees.parliament.uk/committee/52/environment-food-and-rural-affairs-committee/news/217944/.
  106. Bloomberg, report of a revised London & Valley Water offer for Thames Water, 23 September 2026; not read for this briefing.
  107. Thames STW map v0.7 (2 October 2026), built by script from the supporting register; works locations and receiving waters from the Environment Agency, Consented Discharges to Controlled Waters with Conditions (August 2026 extract).
  108. Environment Agency, Consented Discharges to Controlled Waters database (live permits) and public register; EDM storm overflow annual returns 2021–2025; UWWTD population equivalent 2022 (Fairford) and EA inspection reports (others); compiled in the supporting register.
  109. Environment Agency, EDM storm overflow annual returns 2021–2025, Thames Water sheets, Fairford STW (counted on the 12/24 method).
  110. Supporting register.
  111. Environment Agency, Water Industry National Environment Programme (WINEP3) national dataset, 27 March 2020, action THM00095 (Fairford STW, driver U_IMP5).
  112. Environment Agency, permit CATM.3517, variation issued 17 June 2025, schedule 3 table S3.3.
  113. Environment Agency compliance assessment report C0004658, Fairford STW, 30 December 2025; actions C0004658-A01 and -A02.
  114. Thames Water, AMP8 Delivery Plan data tables v3 (13 August 2026) and earlier version (24 December 2025); supporting late-upgrades workbook, Fairford row.
  115. Environment Agency compliance assessment report R/583695, Bibury WwTW, review of DWF exceedance investigation report, 9 October 2025.
  116. Environment Agency, EDM storm overflow annual returns 2021–2025, Andoversford STW.
  117. Environment Agency, Consented Discharges database, permit CNTD.0001 (live version; variation effective 7 May 2021); the Agency’s inspection reports give 2.3 l/s.
  118. Supporting register; Environment Agency compliance assessment report C0011272, Andoversford WwTW, 12 May 2026.
  119. Supporting register: population equivalent (UWWTD 2022), permitted FFT 3,470 l/s, required 4,266.5 l/s.
  120. Thames Water, AMP8 Delivery Plan data tables v3, 13 August 2026, table DPWW4, as transcribed in the supporting register; £m as published, output-only basis, price base to be confirmed.
  121. Thames Water, “Investment plans for storm discharge sites”, seven versions May 2023 – April 2026, as transcribed in the supporting register.
  122. Environment Agency compliance assessment report R/567104 (Maple Lodge, permit CNTD.0071), 9 July 2025; transcribed in the supporting register.
  123. Supporting register: permitted FFT 33.5 l/s against 51.8 l/s required; EDM spill hours 2024 and 2025.
  124. Environment Agency compliance assessment reports I/539552 and I/548130 (Markyate, permit CSSC.8549), assessment 23 January 2025; transcribed in the supporting register.
  125. Supporting register: permitted FFT 353 l/s against 428.5 l/s required; EDM spill hours 2024 and 2025.
  126. Supporting register: permitted FFT 316 l/s against 327.2 l/s required.
  127. Environment Agency compliance assessment report C0003253 (Berkhamsted, permit CATM.3074), 25 November 2025; transcribed in the supporting register.