That is indeed correct, Dizzee Rascal, bonding is legally not enforceable, although I know that a lot of units like to think it is and make you sign a piece of paper. I suppose ATC it is a small world and word would get around quickly if you were to leave before your 'bonding period' runs out without paying up...
However, as an aside, NATS don't bond you because they know it is not legal and couldn't be enforced.