I had a training bond when I left the airline 4 years ago (5 years or 28.000 EUR). When I left the airline after 3 years they wanted the money. I went to out to seek some legal help and it turned out that training bonds to start with are not legal in my country.(EU member state)
So one simple letter from a law firm was enough to put an end to the story.
I dont know how things are in the UK. You should go and seek some local legal help.
LC