This happened to my sister and it took the bank months to sort out (although as per the above answer they advised it would only take 3 weeks). This was for several hundred pounds and the bank only provided an overdraft for the amount when she involved our local MP.
As a suggestion, if you experience difficulties in recovering your money, you could ask the bank if there is a CCTV camera in the cashpoint (some cashpoints capture each transaction) if so you can request a copy of the footage of yourself under the 91 Data Protection Act, and the bank would have to provide the data within 40 days, you need to do this ASAP as they may not keep the recordings for too long (often only 30 days) this would certainly assist you in proving your case. Also there may be CCTV cameras in the area (e.g. if you were in a shopping centre) and you can also request this footage too, from whoever makes the recording. Not easy to do but may be useful in a worst case scenario.
My only other advice is persistence, put everything in writing, ask for copies of any forms you complete for the bank, keep a record of names/times/dates of any contact you have with the bank or that they have with you and don't let them fob you off.
Good luck!