Oh, I asked further up if you're going to be holding on account for reuse later, to which you confirmed yes.
If she's paid you £345 (the value of the invoice) and you're refunding £355 (i.e. paying the funds to her bank) then this is very simple.
All you need to do is go into the screen to create a new invoice and then enter a negative amount for £355. QuickFile will then assume this is a refund and ask you which bank account you want to refund from.
Something like this:
