Five minutes of preparation changes what a plumbing visit costs, and one of those minutes is legal rather than practical.
Find the stopcock before you need it
This is the best-evidenced money-saving step in domestic plumbing, and Which? makes the argument in cost terms directly: 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.
Your internal stopcock is usually under the kitchen sink, and otherwise in a downstairs loo, a utility room or a cupboard. The external stop tap sits under a small square metal cover outside. Test the internal one at least once a year by turning it slowly clockwise, then opening the nearest cold tap to confirm the flow has stopped.
If it is stiff, stop. Older brass stopcocks seize with limescale, and a seized spindle snaps cleanly when twisted hard. Discovering that during an emergency is the most expensive sequence of events available to a homeowner, and it starts with a small effort to force something.
Decide honestly whether this is an emergency
Urgency is the largest controllable number on a plumbing bill. A booked visit carries a call-out charge of around £60 against £110 to £180 same-day or emergency, and out-of-hours labour can run to twice the normal hourly rate.
The working test is containment. A leak you can isolate at the stopcock, or catch in a bucket, is a scheduled job. One you cannot isolate, or one anywhere near electrics, is not. A dripping tap is never an emergency however loud it is at three in the morning.
Photograph it and read the label
A photograph of the leak, the tap, the joint or the wet patch does more than any description, and a plumber can often tell from an image whether the job needs a washer, a cartridge or a new tap.
Note the make and model as well. Boilers carry a data plate with the model and serial number, and taps are worth identifying by type: separate hot and cold handles that turn several times have washers, while quarter-turn and single-lever mixers use ceramic cartridges instead. On the London figures that single detail moves the bracket by roughly £20 to £35.
Clear the way and isolate what you safely can
Whatever is under the sink is going to come out, and it is cheaper for you to do it than for someone billing by the hour. Empty the cupboard, move what is in front of the boiler, and shut the pets in another room.
If there are isolation valves under the fitting, turning those is better than shutting the mains, because the rest of the house keeps working. If there are none, the stopcock is the only option, which is another argument for having tested it.
Write the whole list down
The travel, parking and call-out are paid once regardless. A second and third small job added to the same visit cost close to labour alone, so a stiff stopcock, a slow-filling cistern and a dripping tap booked together cost a fraction of three call-outs.
Say all of it when you ring, not when they arrive. A plumber who knows there are three hours of work coming can price the visit as a block, and a block price is usually the cheaper basis.
Your cancellation right works differently for an urgent repair
Here is the part almost nobody publishes, and it is worth knowing before you agree to anything at your kitchen table.
A contract made in your home is an off-premises contract, which normally carries a 14-day cancellation right under the Consumer Contracts Regulations 2013. But regulation 28(1)(e) disapplies that right where you specifically requested a visit for the purpose of urgent repairs or maintenance. Call someone out to a burst pipe and you cannot cancel the repair you asked for, which is fair enough.
Regulation 28(2) is the part that matters. Where, during that visit, the trader supplies services beyond those you specifically requested, or goods other than replacement parts necessary for the repair, the cancellation right does apply to those additional services and goods. The emergency leak is locked in. The new boiler, the bathroom refit or the service plan agreed while someone is standing in your kitchen is not.
So if a repair visit turns into a conversation about a much larger job, you have 14 days on the larger job. Say yes to the repair and take the rest away to think about.
Have the questions ready
What is the call-out charge, does it include any labour, what is the hourly rate afterwards, and is that with or without VAT. Ask them before describing the problem: a plumber who prices the job rather than the visit will say so, and that is a better answer than any of them.
Preparation is the only part of a plumbing bill you control entirely.