If you pay them enough they wont try to nick your work.
The balance is to pay them sufficient. I've never hid my prices from staff as they are on a % they can wok it out anyway.
Its best to be transparent with them.
I've never had a problem with this method.
Regarding IR rules, if you provide more than 60% of there income, you have to employ them.
The way we get round this is that my work partner has work of his own, when we do his he pays me a %, the same I pay him. it gives me a breather from being boss, and i get my money on the night (usually about £90)
When i'm boss he gets the cash but I get a greater income.
It all boils down to trust, and what you charge.