Contact them and ask for a refund.
Barclays have refunded me in the past as it was the only time I'd gone over in more than a year.
Just phone them and tell them it was a genuine mistake and you feel that a charge of that amount is unreasonable.
Be firm but polite and ask to speak to a manager if you don't get anywhere.
Barclays have reduced their charge to £8 from £35 seemingly as a result of all the issues in the press about charges.
Tell the bank that if they won't refund it you will take the matter to the banking authorities as it is unfair and disproportionate.
Good luck!