It's not just an expense though you'd be paying someone and therefore you should be deducting their tax.......I think!
When I did a tax course, which was a waste of time lol, I was told that you could use a subbie twice in a year. Anymore than that, they were to be treated as an employee... But out in the real world, it doesn't work like that. I do a lot of subbie work, and different builders work in different ways, because that's how their accountants say to do it. The guy I'm working for at the moment likes to be invoiced, so he's not paying the tax, the one before used to pay the tax on the whole lot, then transfer my wages into my bank account, and I'd pay tax on that separately.. The one before that would get the customer to pay me directly, or pay me cash!?!
All I know, is I'm paying the tax on what I receive, so I should be ok.. If they're doing it wrong, it's between them, their accountants, and the tax man.