Thanks for your advice.
I do online banking, and don't get statements, but I do get letters saying 'you have gone overdrawn, we will charge you' but they don't say how much. I'm *hoping* it will only be 60 pounds this time, not 90, but I'm not absolutely certain and the letters don't say how much you'll get charged, so I'm budgetting for 90 just in case. I wouldn't want to put the amount on the claim letter - if its wrong then that might lead to trouble with the rest of the claim.