This is a massive concern and argument we are about to embark on with HMRC actually.
HSE say that employers should pay for the training of their PAYE staff. As an employer, if you pay for training their is tax relief available. There are also grants within CITB (by paying levies). As an NASC member, you are obliged to pay for training.
There is also something called salary sacrifice, whereby the employer pays for the course, gets the tax relief but deducts the cost of what is actually paid (course - tax benefits) from employee's pay. This is because PAYE workers cannot claim for training costs - which we find outrageous!!
Now I am thinking that if said employee pays for his own course, that HSE say his employer should pay for, and so reports said employer to HSE - I doubt he'd be in his job for long... So you just pay anyway.
However, by the very nature of the construction industry and the way many of you have to work, often moving around and not being with an employer for that long, it then does become a massive investment for the employer to put you through training if you are then going to up and leave... even within a couple of years.
A smart employer would definitely have a contract with that sort of clause in it... why should they pay for your training? However, if the government made the system fairer and recognised that so much training within construction is mandatory these days, that training is not about "betterment" but much emphasis on H&S and so pass on the tax relief to all workers whatever employment status, at least it would help your pockets out a little.
So, in short, I think the employer should be able to claim back some of that expense if you leave very soon after training (bearing in mind part 1 takes at least 6 months?)