Payment in plumbing splits cleanly by size of job, and the sensible arrangement for a blocked sink is the wrong one for a boiler.
Repairs: on completion, against an invoice
For anything that takes an afternoon, the normal arrangement is payment when the work is finished, by card or bank transfer, against an invoice showing labour, the call-out, parts and VAT separately.
There is usually nothing for a deposit to fund on a repair. The parts are a few pounds and already in the van. A request for money up front on a tap or a blockage is worth a question, not because it is necessarily improper, but because the usual justification for a deposit does not exist.
Deposits, where they do apply
No UK law caps deposits at any figure. The 25% ceiling that circulates is Citizens Advice guidance, and the FMB independently gives the same number. Two bodies agreeing is stronger than either alone, but it is still guidance rather than statute.
What is defensible is the framing both use: a legitimate upfront payment is a contribution towards materials rather than a payment for labour that has not happened. On a bathroom or a boiler, where several thousand pounds of goods are ordered before anyone arrives, that is a real cost. On a repair it is not.
Record any deposit in writing, and pay it in a way that leaves a trace.
Installations: stages tied to work you can see
On an installation running to several thousand pounds rather than a repair running to a few hundred, the arrangement to ask for is staged payments tied to completed, verifiable stages, with the final payment falling due on completion after snagging.
Paying in stages is the mechanism that keeps problems fixable. A payment schedule that front-loads the money removes your only practical leverage at exactly the point you might need it.
The card rule worth knowing
This is the most valuable single fact on the page, and it is widely misunderstood.
Under section 75 of the Consumer Credit Act 1974, where you pay by credit card the issuer is jointly and severally liable with the trader for misrepresentation or breach of contract. The thresholds in section 75(3)(b) attach to the cash price of the job, which must be over £100 and no more than £30,000, and not to the amount you put on the card.
There is no minimum card payment. As the Financial Ombudsman puts it, section 75 applies even where only part of the payment was made using credit. So a small deposit on a credit card protects the whole contract price within that band, and it protects it even if the business ceases trading, which is when the protection is worth most.
A debit card does not do this. What it gives you is chargeback, which is a card scheme process rather than a legal right, with a much shorter window and no obligation on the bank to raise it. For an installation, the credit card is the better instrument.
Paying in cash
Paying cash is legal and the UK sets no limit on what a consumer may pay in cash. A discount for cash is lawful too: HMRC's own manual confirms VAT is due on the lower sum actually paid.
The line sits at the paperwork rather than the discount. A lower price with an invoice is fine. A lower price conditional on there being no invoice is an offer to evade tax, and the plumber is the one committing the offence. A homeowner who pays cash and takes a receipt has done nothing wrong.
A plumber charging no VAT usually has an innocent explanation. The registration threshold is £90,000 of taxable turnover, so a small sole trader below it legitimately charges none.
What cash costs you is enforcement rather than rights. Your Consumer Rights Act protections do not depend on how you paid, but cash leaves no proof of payment, no Section 75, no chargeback, and a much harder time proving a date if a guarantee is ever tested.
Agree the basis before the work
Section 51 of the Consumer Rights Act 2015 gives you a reasonable price and no more, but only where no price was agreed at all. Once a figure has been agreed, that figure is what you owe.
So the conversation that matters is the one before anyone opens a toolbox: what the call-out is, whether labour sits inside it, whether the price covers VAT, and whether you are being given a fixed figure or an indication.
Get a receipt whatever the method, because it is the document every later argument depends on.