Thames Water Leak Letter, Section 75 and Your Four Weeks

What the letter actually means

Thames Water put it plainly on their own help pages: you may receive a letter or email from them if they detect a leak at your property. The detection is not someone visiting. It is the meter. A traditional meter gets read and compared against your own history and against what a property of your size normally uses. A smart meter reports flow continuously, and a flow that never drops to zero through the night is the classic signature of a leak rather than a busy household.

It is worth being clear about what the letter is not. It is not a fine, it is not a bill, and it is not an accusation that you have been careless with water. It is a notification, and its real significance is that it starts a clock. The date on it is the date Thames Water counts from if the leak is still running in four weeks, and it is the same date the leak allowance rules are measured against.

• Continuous flow recorded overnight with nothing switched on
• Consumption well above what the property normally uses
• A smart meter alert rather than a manual reading
• Delivered as a letter, an email, or a card left at the property
• No indication anywhere in it of where the water is going

Confirm it yourself in thirty minutes

Before you call anyone, check whether the meter agrees with the letter. Turn off every tap and every appliance that uses water, including the dishwasher and the washing machine, and wait for any toilet cistern to finish filling. Read the meter, write the figure down, then leave the property untouched for thirty minutes and read it again.

If the reading has moved, water is leaving the system with nothing calling for it and the letter is right. If it has not moved at all, either the leak is intermittent or something else triggered the alert, such as a genuine spell of heavy use or a misread meter. Either way you now have a fact rather than a worry, and it is the first thing we will ask you on the phone.

Section 75 and the four-week rule: what happens if you miss it

Thames Water's position is not ambiguous: you must fix it within four weeks. If the repair has not happened by then, Section 75 of the Water Industry Act 1991 lets them carry out the work themselves and charge you for it, and in an emergency they can turn the supply off until the leak is repaired.

The cost of missing the window is not only the repair. A leak fixed under Section 75 is specifically excluded from the leak allowance, so the water you lost stops being recoverable at the same moment the repair stops being yours to arrange. Four weeks sounds generous until two of them have gone on waiting for callbacks.

• Repaired inside four weeks: you choose the contractor and can claim for the water lost
• Repaired late: the allowance is at risk and enforcement is still open to them
• Not repaired: Thames Water can do the work under Section 75 and bill you for it
• Not repaired and urgent: the supply can be turned off until the leak is fixed

Whose pipe is it, yours or theirs?

The boundary is the stop tap at the edge of your property, usually under a small cover in the pavement or the front path. Everything from the water main up to that point belongs to Thames Water. Everything after it is yours, and their own guidance spells out what that includes: the internal plumbing and the supply pipe under your garden and driveway.

This is why a high usage letter so often turns into an excavation conversation. The supply pipe is the one length of your plumbing that is buried, unseen and usually old, and on a London property it can run under a paved front garden, a shared path, or a driveway laid over it decades after the pipe went in. Establishing that the leak is on that pipe is straightforward. Establishing where on it, without opening the whole run, is the actual job.

What we find when we survey

Across our last 400 leak detection surveys we found the cause of the leak in 95 per cent of them. In another 2 per cent the water was coming from a neighbouring property rather than the one we were called to. In the remaining 3 per cent it was not a plumbing leak at all: gutters, roofs and other structural defects letting water into the building. The last five per cent were not unsolved, they were simply not the client's own pipework.

We publish that split because those cases are where money gets wasted. An owner certain the leak is theirs can spend weeks and a serious amount of money chasing pipework that was never leaking, when the evidence pointed next door or at the roof within the first hour. Behind a Thames Water letter the odds tilt further towards your own supply, because a meter recording continuous flow is measuring water passing through your pipe. The discipline is the same either way: confirm the source before anyone opens anything up.

How the leak is located without digging up the garden

The order matters. We start at the meter and isolate. Closing the internal stop tap and watching whether the meter keeps moving tells us straight away whether the leak is inside the building or out on the buried supply pipe, and that single test decides which half of the property the rest of the visit is spent on.

For a buried supply pipe we listen. Acoustic ground microphones pick up the sound a pressurised leak makes as water escapes into the surrounding ground, and correlation equipment compares that sound at two points along the run to place the leak within a short distance. Where the pipe is plastic, which deadens the sound, we drain it and introduce a hydrogen tracer gas that rises through the soil and registers at the surface directly above the escape point.

Inside the building, thermal imaging reads the temperature difference a leak leaves behind a wall or under a floor, and calibrated moisture meters map how far the water has actually travelled, which is often nowhere near where the stain appears. The purpose of all of it is the same: mark a small area with confidence rather than open a large one on a guess.

• Meter isolation test to separate internal plumbing from the buried supply
• Acoustic ground microphone and correlation along the supply pipe run
• Hydrogen tracer gas where plastic pipework deadens the sound
• Thermal imaging across walls, floors and heating runs
• Moisture mapping to show how far the water has tracked
• A marked location and a written report before anything is opened

The leak allowance, and the paperwork it needs

The leak allowance is a rebate for the water you lost, not for the repair. Thames Water's conditions are specific: the property has to be metered, the leak has to be repaired within four weeks of them first notifying you, the claim has to reach them within three months of the repair date, and the leak must not have been fixed under Section 75.

They also want evidence, which is where the contractor you pick makes a difference. The claim needs the plumber's details and invoice, or a Thames Water job number if they carried out the repair themselves, together with meter readings from before, during and after. We record the meter at the start and the end of the visit as a matter of course, and the invoice and detection report are written to be forwarded straight on rather than explained.

• The property must be on a water meter
• Repair completed within four weeks of the notification
• Claim submitted within three months of the repair date
• Plumber's details and invoice, or a Thames Water job number
• Meter readings from before, during and after the leak
• Not eligible where the repair was carried out under Section 75

What it costs, and what happens if there is nothing to find

Buried supply pipe and mains work is charged at £200 an hour and a typical trace takes two to three hours, so £400 to £600 in total. Detection inside the property is £160 an hour and usually runs two to four hours, so £320 to £640. You are told the hourly rate before we travel and it does not move on the day, whatever the leak turns out to be.

If we attend a confirmed live leak and cannot locate it, the detection time is not charged, however long we spent looking. That guarantee is the part that matters on a job with a deadline attached to it, because a cheap hourly rate with no such promise can run for an afternoon and still leave you holding a letter, a bill and an unfound leak.

Who you are actually calling

London Leak Specialist Ltd is a company registered in England and Wales, number 17437507. The name at the top of this page is the company's own registered name rather than a trading style bolted onto a landing page for the search term, and the engineers who attend are ours.

We are independent and are not affiliated with Thames Water, and nothing here should be read as speaking for them. The rules quoted on this page are theirs and are published on their own website. The survey, the report and the no find, no fee guarantee are ours.