davo, fair play for hanging in there your figures do seem ok. But like williamx says, your figures do seem to be ok on the basis of you working it as a sole trader. Your expenses are almost identical to mine, infact a little more so you have some scope there.
Though you have missed out employers liability (i was quoted £1184 last year) and the other expenses of having an employee, unless you have a way around that which others like me don't know about. Unless its cash in hand for you.
You have nothing to lose really as you have an income independant of window cleaning, so what the hell, why not go for it?
I dunno why people have got upset with your original post? Because other people on here have posted about taking work off other window cleaners and apart from the taboo of undercutting to get it, the general consensus seems to be that is business.
It's hard to run any business where you don't know the ins an outs of the service but you do seem to have put alot of research into this. Although i do agree that you will need to get your hands dirty. I hope you keep posting on your progress as it's very bold to go straight into it as an employer.
But who dares wins and all that.