Most of what separates a good plumbing booking from a bad one is settled on the phone, before anyone gets in a van. Four questions do the work, and none of them is about price directly.
Are you Gas Safe registered, and for which work?
Ask this first, and ask it even if the job seems unrelated to gas.
Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 restricts work on gas fittings and appliances to members of a class of persons approved by the Health and Safety Executive, which is the Gas Safe Register. It is the one statutory credential in plumbing. Everything else in the trade is voluntary.
The distinction that gets lost: a plumber is not automatically Gas Safe registered. Plumbing and gas are separate competencies, reached by separate training, and plenty of skilled plumbers hold no gas qualification. Registration is also granted by work category, so being registered for one type of appliance does not mean being registered for all of them.
Ask for the registration number and check it at gassaferegister.co.uk. A registered engineer carries an ID card, and the register is the place to confirm both the licence and what it covers.
Unvented hot water cylinders are regulated on a separate footing, under requirement G3 of the Building Regulations, which is a building control matter rather than a criminal registration one. If the job involves one, ask what qualifies them to touch it.
What is the call-out charge, and does it include any labour?
A booked, non-urgent visit runs around £60 on Checkatrade's scheduled-visit figure. A same-day or emergency one is £110 to £180. Out of hours, the hourly rate on top can double.
Whether the first hour of work sits inside that charge varies by plumber, and no authoritative source states a norm. That is not a detail: it is the difference between £60 covering an hour and £60 plus £65 for the hour, decided by which number you dialled.
Is that inclusive or exclusive of VAT?
Published cost guides headline figures excluding VAT. Homeowner quotes normally include it. A rate quoted on one basis and compared against the other is 20% wrong before anyone opens a toolbox, which on the numbers above is more than the gap you are trying to judge.
A plumber quoting no VAT at all usually has an innocent explanation: the registration threshold is £90,000 of taxable turnover, and a small sole trader below it legitimately charges none.
Is this a fixed price or an indication?
The trade treats a quote as a fixed figure and an estimate as an indication, but that distinction is convention rather than law. No statute defines either term, so the label on the document settles nothing.
What does matter is whether a price was agreed at all. Section 51 of the Consumer Rights Act 2015 only bites where none was, and then you owe a reasonable price and no more. Once you have agreed a figure, you owe that figure. Getting a number in writing before the work is the whole of your leverage.
Who supplies the parts, and what happens to the price?
Housekeep notes that plumbers commonly add 20 to 25% on materials. That is a single source without published methodology, so treat it as a reason to ask rather than a rate to expect.
Supplying your own parts is often cheaper and carries one consequence worth understanding: if the part fails, the workmanship remains the plumber's responsibility and the product becomes yours.
What insurance do you carry, and do you employ anyone?
Employers' liability insurance is compulsory under the Employers' Liability (Compulsory Insurance) Act 1969 for any firm with staff, at a statutory minimum of £5 million, and a sole trader with no employees falls outside it. Public liability insurance is not legally required at all, and HSE describes it as generally voluntary, so a plumber without it is a poor risk rather than a criminal.
For water damage inside an occupied house, public liability is the cover that responds. Ask what the limit is and whether the certificate names the exact business you are contracting with.
What happens if you find something else?
On South London housing stock this is not hypothetical. A seized stopcock turns a tap job into a stopcock job at £140 to £240, on a basis the guide leaves unstated for VAT. Corroded brass in a Victorian conversion means pipework cut back and remade. A tap that still drips after a new washer usually needs the seat re-cut or the tap replaced.
Ask what the rate would be for the extra work and who decides it is needed, because that conversation is far cheaper at the kitchen table than on the phone afterwards.
And the one that reprices everything
Ask what it would cost to do everything on your list in a single visit. The travel, the parking and the call-out are paid once, so the second and third small jobs cost close to labour alone.
Ask the same set of every plumber you ring, and the answers become comparable.