Nobody has surveyed British plumbers about what they wish homeowners understood, so what follows is not folklore attributed to a trade. It is the set of things the published evidence supports, which is a smaller and more useful list.
Where your stopcock is, and whether it turns
The best-sourced item here, and the one with money attached. Which? puts it in cost terms: if the water cannot be shut off, the plumber may have to drain the whole system before repairing anything, which takes time and often means higher charges.
The corollary is the bit people miss. Test it gently and in advance, because older brass stopcocks seize with limescale and a seized spindle snaps cleanly when forced. A stopcock discovered to be broken during a leak has turned a repair into an emergency, and emergencies are the dearest thing in this trade.
An hour is the most expensive way to buy their time
A day of plumbing costs materially less per hour than an hour of it does, around 13% less on Checkatrade's own published figures, which the answer on day rates works through.
That gap is not a mistake. It is the overhead spread over eight hours rather than one: travel across South London, parking a van in a controlled zone, loading and unloading, and the quotes that never became work. Booking longer costs less per hour, and a homeowner who understands that stops asking for the hourly rate and starts asking for a price to do the whole list.
Urgency is the lever you control
A booked, non-urgent visit carries a call-out charge of around £60. A same-day or emergency one is £110 to £180, and out-of-hours labour can run to twice the normal rate.
The same person, the same address, the same repair, at three times the price, decided entirely by which day you choose. Where the water can be isolated, choosing Tuesday morning over Sunday night is the largest single saving available on a small plumbing job.
The part is not the bill, and that is not a rip-off
On the smallest jobs, better than nine tenths of the invoice has nothing to do with the component, and the arithmetic is in the answer on why plumbers are so expensive in the UK.
What it buys is an attended hour rather than a component, which is the pricing logic set out in the answer on why plumbers are so expensive in the UK. It is why the smallest jobs look worst on paper and are in fact priced correctly.
A plumber is not automatically a gas engineer
Only a Gas Safe registered person may lawfully work on gas fittings and appliances, and being a plumber does not confer that. The answer on what questions to ask a plumber covers why, and what to ask.
What plumbers report is that homeowners hesitate to ask, as though the question implied distrust. It does not. A good one would rather be asked than assumed about.
Clearing the cupboard is billable time
Whatever sits under the sink is coming out, and someone is being paid by the hour to do it. So is moving the boxes in front of the boiler, and so is waiting while a cat is caught.
None of this is a complaint about tidiness. It is that access is time and time is the entire product.
Tell them everything on the first call
A plumber who arrives expecting a washer and finds a ceramic cartridge is having a conversation about price at your kitchen sink, which suits nobody. The same call handled properly beforehand produces a firmer number and a van carrying the right parts.
The corollary is the list. Mentioning the slow cistern only after the tap is done means either an unplanned extra half hour or a second visit, and the second visit carries its own travel and its own call-out.
Some jobs are yours
Bleeding a radiator, plunging a sink, clearing a shower trap, topping up boiler pressure through the filling loop. A plumber telling you to do one of those has just declined an hour of paid work, which is worth noticing when you decide who to call next time.
The things worth knowing are the cheap ones, and most of them happen before the phone call.