I got a similar letter a while ago also demanding money within 14 days. I spoke to a few people about it, including ALPASA and they all told me the same thing,"You signed the training bond so PAY!" So I paid. Rossairs lawyers gave me lots of hassles and because I could not come up with the money within the 14 days, even though I had spoken to them about that, they still caused big trouble for me.
So I would advise paying it. It is the easiest way to get rid of them. Unless you are keen on a fight. If you are good luck.