The useful question is not which plumber is best. It is which register answers the job you have, because only one of them carries legal force.
Gas work has a statutory answer
Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 restricts work on gas fittings and appliances to a class of persons approved by the Health and Safety Executive, which in practice means the Gas Safe Register. Anyone outside it working on your boiler is committing a criminal offence.
Search the register directly at gassaferegister.co.uk rather than trusting a logo. Registration is granted by work category, so the register tells you not only whether someone is on it but what they are signed off to touch.
The point that catches people out is the reverse direction: being a plumber does not make somebody Gas Safe registered, and the answer on what to ask a plumber sets out what follows from that.
Everything else in plumbing is voluntary
There is no plumbing licence. Anyone may trade as a plumber tomorrow with no qualification, registration or insurance, entirely lawfully. What exists instead is a set of voluntary schemes, and knowing what each one is for saves you from treating a badge as a guarantee.
- WaterSafe (watersafe.org.uk): an umbrella list of plumbers approved under the water industry's contractor schemes, which is the relevant one for work on the supply side of your system.
- CIPHE (ciphe.org.uk): the Chartered Institute of Plumbing and Heating Engineering, the profession's chartered body. Membership is an individual professional credential rather than a business vetting.
- APHC (aphc.co.uk): the Association of Plumbing and Heating Contractors, a trade association for firms.
- TrustMark (trustmark.org.uk/homeowner/find-a-tradesperson): the Government Endorsed Quality Scheme, which requires registered businesses to carry financial protection covering prepayments and deposits. It is an umbrella rather than an inspectorate: TrustMark does not vet tradespeople directly but licenses around forty Scheme Providers who do, and their rigour varies, so the question worth asking is which Scheme Provider registered the business.
None of these is a licence and none of them is required by law. They are evidence, and evidence you can check in a couple of minutes is worth more than evidence printed on a van.
Search the exact company name
Whatever the source of the recommendation, search the name on the quote rather than the trading name on the paperwork. The two diverge more often than people expect, and a business appearing on a register under a different legal entity is worth a question before it is worth a contract.
The insurer behind an insurance certificate can be checked too. The FCA Financial Services Register at register.fca.org.uk confirms whether an insurer is authorised, which is the check that survives a forged certificate.
Personal recommendation, used properly
A neighbour's recommendation is valuable in plumbing, because South London housing stock repeats itself. Someone who has worked on the pipework of a Victorian conversion three doors down knows what is behind your boxing before the boxing comes off.
Ask what the job was rather than whether they were happy. A plumber who fitted a new bathroom is not necessarily the person for a below-ground drainage problem, and the recommendation carries information only when the work matches.
Do not do this during an emergency
The worst possible moment to choose a plumber is while water is coming through a ceiling. Urgency is the most expensive variable in the trade, out-of-hours work runs at up to twice the normal hourly rate plus a call-out charge, and you have no ability to compare anything.
Find someone while nothing is wrong. Get a name, keep the number, and ask a small job of them first if you can. A tap or a stiff valve is a cheap way to discover how somebody works before a boiler depends on the answer.
Three quotes is advice for installations, not repairs
The standard guidance to get three quotes makes sense for a bathroom or a boiler, where the scope is defined and the money is meaningful. It makes very little sense for a blocked sink, where you would be paying three call-out charges to compare prices on an hour of work.
For repairs, the substitute is a phone call that establishes the call-out charge, the hourly rate, whether the first hour is inside the charge and whether VAT is on or off. That is comparable information without three vans in your street.
What to discount entirely
A logo with no register behind it. A five-star average with no addresses attached. And a stranger at your door reporting a problem you cannot see, which is a banned commercial practice rather than merely an unwelcome one.
The best time to find a plumber is a quiet Tuesday when nothing is leaking.