Most of what distinguishes a good plumber from an adequate one is audible before anyone arrives. These are the things worth listening for.
They ask questions before they give a number
A drip from the spout of a traditional tap is a washer. The same drip from a single-lever mixer is a ceramic cartridge, and the London figures for those two repairs are £60 to £95 and £80 to £130 including VAT. A plumber who asks what kind of tap it is before pricing is not stalling; they are refusing to quote for a job they have not identified.
The same applies upward. What make and model is the boiler, is there an isolation valve under the basin, does your stopcock turn. Every one of those answers changes the time on site, and time is what you are buying.
They tell you what they are not registered to do
This is the clearest signal in the trade, and it costs a plumber work to give you.
Gas is legally restricted under regulation 3 of the Gas Safety (Installation and Use) Regulations 1998, and only a Gas Safe registered person may work on gas fittings and appliances. Unvented hot water cylinder work is regulated on a different footing: requirement G3 of the Building Regulations applies to it and it needs a suitably qualified person, but compliance there runs through building control rather than through the criminal offence that gas registration carries. A plumber who says "that is a gas job and I am not registered for it, here is who to ring" has just demonstrated more than any badge on a website. One who takes the work anyway has demonstrated something else.
Being asked for a registration number is normal and expected. A plumber who bristles at it is telling you how the rest of the job will go.
They tell you when you do not need them
Bleeding a radiator, plunging a blocked sink, clearing hair out of a shower trap and changing a washer on a traditional tap with working isolation valves are all within reach of most households, and a plumber who says so is trading a small job now for the calls that follow.
The reverse matters as much. Which? warns that older brass stopcocks seize with limescale and that a seized spindle snaps cleanly when forced, which turns a modest repair into an emergency with the water off. A good plumber will tell you where the line sits rather than either taking every job or waving you off with all of them.
Their quote can be compared with someone else's
Published cost guides exclude VAT and homeowner quotes usually include it, so a figure quoted without stating its basis is a fifth adrift before you start. So is a call-out charge whose relationship to the first hour is left vague, because no authoritative source establishes a norm either way.
A plumber who volunteers the basis, meaning what the call-out is, whether labour sits inside it, whether VAT is on or off and who is supplying the parts, has handed you the ability to shop around. That is a confident thing to do and a rare one.
They price the visit, not just the hour
Longer bookings are cheaper per hour, because the travel, parking and dead time get shared out. The published figures behind that are in the answer on plumbers' day rates.
A plumber who hears your list and offers to do the lot in one visit at a block price is applying that arithmetic in your favour. A plumber who quotes a bare hourly rate and nothing else has left the risk of a slow afternoon entirely with you.
The guarantee under the guarantee
Any promise a plumber makes sits on top of a statutory one. Section 49 of the Consumer Rights Act 2015 implies into every service contract a term that the work will be performed with reasonable care and skill. It is automatic, it cannot be excluded, and it applies whether or not anyone mentioned a guarantee.
If work is defective, section 55 gives you repeat performance at the trader's cost, including labour and materials, but you normally have to give them the chance to put it right before escalating. A plumber who agrees to come back without argument is not doing you a favour; they are doing what the statute already requires, and the good ones know it.
The van, the invoice and the address
A stocked van means the washer sizes, seats and O-rings are already with them and the job finishes on the first visit rather than after a trip to a merchant. An invoice with a business address on it means you can find them in six months. Neither is glamorous, and both are the difference between a repair and an episode.
Competence is quieter than salesmanship and easier to check.