I phoned and queried this, and after a lot of hassle they agreed to refund the charge (though I forgot to ask about interest, and I'm waiting to see if they refund it - if not they will be hearing from me again); their version of this is that they can't guarantee that it will take less than five working days for payment to travel from the bank to their computers and for the amount to be credited. And there was a weekend and a bank holiday in between.
Does anyone else think it odd that they can remove cash from my account in a few seconds, but it takes nearly a week for the electrons to travel from the cashier's computer to theirs - which should be a completely automatic process? Do they shunt into sidings on weekends and public holidays?
It's a couple of weeks till I get the next statement, and if it isn't sorted in full I think that the Banking Ombudsman will be my next port of call. The amount is fairly trivial, the principle isn't.