Hi Lucy
I read somewhere (
http://www.bankactiongroup.co.uk/) that the charges applied to accounts for non payment of direct debits etc may well be illegal under the Unfair (Contracts) Terms Act 1977 and the Unfair Tems in Consumer Contracts Regulations 1999. So it may be worth your while to investigate or get advice and threaten the bank with court action. Get them to show you exactly why it costs so much to 'bounce' something but they very rarely do.
I have a £100 overdraft and was £70 into it, they 'bounced a DD for £20 and charged me £35 for it, this took me over the overdraft limit so they 'bounced' another DD for £5. It took 40 minutes of agruing with them until they could work out the simple maths of it all, if they paid ALL DDs I would still be UNDER the overdraft limit and it was only the charges incorrectly applied which casued the problem, stick with it and you will get the money back, I did.
Good luck to you.