Are you quite sure it isn't a scam? Someone trying it on and hoping you will be green enough to fall for it? If I were you I would be contacting the HR dept where you worked to ask about it. Phone as well as writing, you can answer any questions faster on the phone. However, having a written version on file too is a good thing.
f it is genuine, then it was two years ago and their mistake, so they out you into this debt. You are in a completely different position now, and could be financially quite incapable of repaying it. Citizens' Advice could advise you. They will probably suggest the £1 a week route.