I've not read all the previous replies to this one, so my apologies if these have already been put forward.
1. Write a "final demand" informing him that you will be placing his name on "The National Window Cleaners Debt Register"
Yes, I know, but he doesn't, does he!
2. This second one might be favourite, because the first almost certainly won't work, if he hasn't paid up the now.
Send someone else round there, [preferably large-ish], at a different time to when you normally go, who doesn't look like a window cleaner.
If curiosity gets the better of the bugger, and he opens the door, your mate sticks foot in, [into the door, I mean!], and say's he is your collector.
3. I think I saw this on one of the previous that I did manage to read.
You can, at the end of the day, submit it as a "Bad Debt" on your tax return; I'm just doing my second one from startup, and I've got one for £12 [same house, two cleans].
Baldeagle in Staffs