If an error is made in relation to a direct debit (irrespective of whether that mistake was made by the bank or by the organisation taking the money) the bank is obliged to compensate the account holder (by refunding the money, or in your case removing the overdrawn amount, and by cancelling any bank charges raised as a result of that error):
http://www.thesmartwa...t-debit-guarantee.asp
(If the error was made by the organisation which took money without authorisation it's then up to the bank to try to get the money back from them).
Simply ask, at any A&L branch, for a 'direct debit indemnity form'.
Chris