Hi there, this company dont know for certain that its you they are looking for.
The letter is speculative - someone with your name owes moeny and they know what someone with that name lives at your address. What you need to decide is if it is you that they are looking for, are your prepared to pay this money back? If you cant remember it its likely to be a small amount and I doubt they will demand it all back in one go. If you call, you need to be confident and say this is how much I can afford per week/month and dont try to negotiate too much. The other option is to ignore it! The other option is to get a credit report on your last address and see if there are any negative remarks - this may give you and indication of who it is looking for you and how much they want.