Well, if they have had to send bounce any cheques, or return any direct debits as a result of your being overdrawn, you will get charged anything up to £30 for each item unpaid. You will also be charged anything up to £30 just for being overdrawn.
I had a Business Account with Natwest years ago, and when they failed to credit my account with the cash I deposited the previous afternoon (around 3.5k) they sent several suppliers cheques back and I ended up with over £200 in bank charges. Obviously they had to reimburse me, as it was clearly their fault. But they won't budge if they don't have to.