if the direct debit was an error then you are entitled unde the direct debit garentee to an immediate refund
a few months ago one of my insurances took the whole years payment rather than the monthly one in one go...
i went into the bank, explained it was wrong and that i did NOT authorise that amount of direct debit and the money went back into my account the very same day. It was the BANK that sorted it for me
(the whole amount went back in, not the whole amount - the months amount which was what i expected!)