The VAT rules aren't too bad once you get used to them.
From the UK VAT guide
"14.2.3 Deposits
Most deposits serve primarily as advance payments and will create
tax points under 14.2.2(a) when you receive them."
The general rule is that a tax point (the time at which you need to
account for the tax) is created a) when you issue a VAT Invoice or b)
When you receive payment - whichever happens first.
i.e. you have to account for VAT at the time when the prepayment is
received. This also requires the issue of a VAT Invoice. A credit memo
would not do this.