In late 2024 Ofwat imposed a £122.7 million penalty on Thames Water — the largest in its history — for breaches of its wastewater licence and dividend rules. By April 2026 the story had shifted: reports surfaced of an Ofwat-Thames "deal" that would pause new fines until around 2030 in return for binding investment commitments. For the 360,000 commercial sites Thames serves on the wholesale side, the question is no longer whether the company is in trouble, but what protections business customers have when their supplier of last resort is. The legal answer sits in the Wholesale Retail Code, the Ofwat Customer Protection Code of Practice (CPCoP), and the back-billing limits that apply across the non-household water market.
What the £122.7m penalty actually punished
The Ofwat decision, summarised in its enforcement notices on the regulator's site, broke the £122.7m into two components. The bulk — £104.5 million, equivalent to 9.0% of relevant turnover from Thames' wastewater business — was for failings in sewage treatment works and sewerage networks, the standards for which sit under the Environmental Permitting Regulations and the company's wastewater licence conditions. A separate, smaller penalty addressed dividend payments made when the company's financial resilience did not, in Ofwat's view, justify them. Neither penalty had any direct mechanism to flow back to non-household customers as bill credit. That is what is making business customers, and the trade press, uncomfortable.
The proposed April 2026 enforcement deal — discussed in news coverage in Water Magazine, Smart Water Magazine and elsewhere, and not yet a final regulatory decision at the time of writing — would replace fresh penalties with binding commitments on leakage, infrastructure, and environmental performance. Critics, including the GMB union and several MPs, argue this "rewards failure". Whatever the political view, what matters operationally is that Thames Water remains the regional wholesaler for any business in its area, and your retail relationship is with one of the licensed retailers — Castle Water, Business Stream, Wave, Water Plus and others — covered in our UK business water retailers ranking.
The protections you actually have as a non-household water customer
These come from three places. First, Ofwat's Customer Protection Code of Practice (CPCoP), which all licensed retailers must follow. CPCoP section 9.3.1 caps retroactive billing at 16 months for non-household customers, mirroring the principle behind energy's back-billing rule but with a longer ceiling. If a retailer issues a catch-up bill covering more than 16 months of historic usage, the portion older than 16 months is generally not enforceable — we walk through the mechanics in the water back-billing claims guide. Second, the Wholesale Retail Code (WRC) governs how the wholesaler (Thames) and the retailer split responsibility for meter reads, leak detection and water quality. Where a wholesaler's data is poor, the retailer is required to investigate before billing the customer.
Third, you have the Consumer Council for Water (CCW). CCW reported a 50% rise in water complaints in 2025, with affordability the largest category, and runs a free dispute resolution service for non-household customers up to £25,000. Beyond that, the Water Redress Scheme (WATRS) offers binding adjudication. Neither is widely known among small UK firms. Both are free.
Why the enforcement deal could actually push prices up for business customers
Ofwat's PR24 final determinations, published December 2024, allowed bill increases averaging 36% across England and Wales for the 2025-2030 period. Five companies — Anglian, Northumbrian, South East, Southern and Wessex — appealed to the Competition and Markets Authority. The CMA's final report on 10 March 2026 allowed a further 21% of the £2.7bn the appellants had requested, adding roughly 3% on average to bills already rising 24% under Ofwat's original determination. Thames Water did not appeal in this round, but is widely expected to push for further bill recovery if the enforcement deal locks in mandatory investment commitments. For non-household customers, this matters because wholesale charges flow through to business water bills almost immediately at each annual price reset, with no equivalent of the domestic price cap to soften the impact.
If you operate sites on the Thames network, the practical exposure is that you will see wholesale charge increases through 2026 and 2027 even though your relationship is with a retailer. The retailer's margin is regulated, but the wholesale element is not — it is set through the price review cycle. Understanding which line items on your bill are wholesale vs retail vs network is the single most useful audit you can do this year, and we cover it line-by-line for electricity in our UK business electricity bill walkthrough; the water equivalent follows the same logic.
What to do if this affects you
- Within 30 days: identify your wholesaler and retailer. Your retailer is on the bill; your wholesaler is determined by site location and listed on the Open Water "central market operating system" (CMOS) database, accessible via your retailer.
- Within 60 days: pull the last 24 months of charges and identify any portion older than 16 months that has been billed retroactively. Under CPCoP s.9.3.1 this is generally not enforceable.
- Before contract renewal: ask your retailer in writing for a wholesale-vs-retail charge split, broken out by service. Retailers must provide this information under the WRC. Use it as a baseline for your water cost review against alternative retailers.
- If a leak or wastage event is suspected: insist the retailer raise a "C5 Investigation" with the wholesaler under the WRC. A confirmed leak triggers a leak-allowance adjustment that retailers do not always apply automatically.
- If a complaint stalls: escalate to CCW (free) and, if unresolved, to WATRS. Do not pay disputed amounts beyond the undisputed portion of the bill.
Frequently asked questions
Q: Will Thames Water's penalty reduce my bill?
A: No. Ofwat penalties go to the Treasury, not to customers. Customer redress is delivered, if at all, through performance commitments in the next price control period or through Ofwat's outcome delivery incentives.
Q: Can I switch wholesaler?
A: No. Wholesale provision in England is geographically determined; only the retail layer is open to competition. You can switch retailer.
Q: Does CPCoP s.9.3.1 apply if the retailer issued estimated bills throughout the period?
A: Yes. The 16-month limit on retroactive recovery applies regardless of whether the bills issued during that window were estimated or actual; what matters is when the corrected, accurate bill is presented.
Q: My charges jumped sharply in April 2026. Is that the CMA decision?
A: It can be, if you are on a network operated by one of the five appellant companies. Thames customers are not affected by the CMA appeal, but are affected by the underlying PR24 final determination already in force from April 2025.
Sources: Ofwat enforcement notice on Thames Water, total penalty £122.7m; Ofwat PR24 Final Determinations (December 2024); CMA Provisional Redeterminations report dated 10 March 2026 (water price controls 2025-2030); Consumer Council for Water annual complaints data 2025; Water Magazine coverage of Ofwat-Thames enforcement deal proposal, 7 April 2026.