Ztech,
Are you employing him above board? Have you purchased employer's liability insurance and will you be paying their tax and NI contributions?
Or, are you taking him on as a 'self-employed' contractor; which I belive is illegal if they don't use their own tools, work to their own time, have their own public liability insurance and probably a few other indicators I can't think of.
I think as the law stands, if it's the latter, you could be in for some trouble. But I know it's common practise, not just amongst window cleaners either. I was talking to a painter and decorator recently who 'employed' two lads on a 'self-employed' basis.
I don't think he realised that if either of those lads were tax-investigated; he could be held responsible for paying their tax and national insurance and in-line for a tax investigation himself.
It's no good saying, 'but I only earn £20,000 per year', when you've been caught with two 'employees'.