"Its for a self employed lad we have, so he does his own National Insurance etc"
So why do you stop him 20% of his gross earnings? If he is a "self-employed lad that you have" is he genuinely self-employed (as a sole trader, perhaps?). If so, he is responsible for paying not only his National Insurance but also his income tax. You have no responsibility to deduct tax from his earnings. If a self-employed plumber does £1,000 worth of work for me I don't think he'd be too impressed if I only gave him £800 and promised to pay the other £200 to the taxman for him.
I think you may need to look into this a bit deeper.