These things are all well and good up to the point of a tax inspection. At that point you may find yourself in some bother.
My understanding is that the subbie needs to use his own equipment and decide for himself how much he does and when (as opposed to being 'FINED' for not completing)
As for the time they work for you as long as they have time to do their own they can be 'self employed' if they dont bother then thats not your concern.