Sounds like they might come after you so don't ignore them but buy yourself some more time. I would send a return letter asking for exact dates and times of when the hours worked are in question if they haven't done so. If they did that already I would still send some kind of letter trying to buy yourself time like you have to discuss this with a lawyer or maybe some government labor regulartory agency (here in the states we have the Dept. of Labor as well as several at the state level). You might even find that you aren't responsible for paying them back if you find something to help you.

Good luck finding a job.