Simon, without knowing your personal circumstance it is against the law for them not to pay you, as the company you work for would need to insure you whilst on their premises, so you would essentially be an employee, therefore would require payment, minimum wage?
From what you say, it sounds like they are using you as slave labor by cancelling the other guy for you as you are 'free' which is not on. They could be using the apprentice thing as 'carrot in front of the donkey' too.
On the flip side if you are looking for work, you are 90% more employable if you are employed, although this doesn’t sound like an employment to me.
Get yourself down an employment agency, most of them are pretty good and will do the donkey work in finding you a job, plus you will get paid!