Plastering sits at the low-risk end of the trades for payment disputes, mostly because the jobs are short and the materials are cheap relative to the labour.
The normal pattern on a small job
For a single room, most plasterers buy the materials, do the work over a day or two, and invoice on completion. Bags of finish plaster and a few boards represent a small fraction of the total, so there is little reason to ask you to fund them.
If someone wants a substantial payment before starting a one-day job, ask why. It is not the norm.
When a deposit is reasonable
On a whole house, or where boarding out means a genuinely significant materials order, a deposit covering materials is fair. The plasterer is otherwise financing several thousand pounds of your project on their own account.
A reasonable structure is a materials deposit at the start, a stage payment if the job runs over a week or two, and the balance on completion once you have inspected the work.
What is not reasonable is paying the full amount, or most of it, before any work has been done. Whatever the reason offered, that arrangement leaves you with nothing to hold back if the finish is poor.
Inspect before the final payment
This is where plastering differs from most trades, because of how the defects show.
Look at the walls in daylight, from an angle, not straight on. Raking light reveals ripples, trowel marks, hollows and high spots that are invisible under a ceiling light or head-on. Run your hand over the surface as well.
Check the corners and edges: internal angles should be crisp and straight, external corners should be true, and the line where the wall meets the ceiling should be clean.
Do this before the final payment, and ideally while the plasterer is still there. Genuine defects are far easier to fix on the day than after the plasterer has moved on to the next job.
Bear in mind the wall will still be brown and damp-looking. That is normal and not a defect. You are judging flatness and finish, not colour.
Paying in cash
It is legal to pay a tradesperson in cash, and many plasterers prefer it for the same cash-flow reasons any small business does.
Be clear about what a cash discount actually buys, though. Without an invoice you have no documentary evidence of the contract, the price or the date, which matters if a dispute arises later. You lose the protection of paying by card or bank transfer, which leaves a traceable record. And a discount specifically conditional on there being no paperwork is usually about VAT or tax rather than saving you money.
Ask for an invoice whatever the payment method. A tradesperson who will not provide one is telling you something.
Guarantees
Plastering guarantees are less formal than in trades with registration schemes. Most plasterers will return to make good genuine shrinkage cracking or a scrim joint that has telegraphed through.
What they will not, and should not, cover is cracking caused by structural movement behind the plaster, or damage from you painting before the wall had dried. Agree in advance what happens if cracks appear, and keep the invoice.