Gazza,
Some folks don't read old threads that go into a couple of pages (etc), therefore I suggest you start a new thread titled something like'Unofficial Clean it Up Members and Anyone Else for a window cleaning 'knees-up''.
Try another forum too, and the new one as well. Get some numbers.
And go from there. Get enough replies; get a deposit to show they're serious about going (£10 to £20) and plan from there.
Obviously if you only get paid 10 deposits that's going to change your plans from the hundred or so members that say, 'Yes, please'.
Why not just ask for a cheque and don't cash them till you have the minimum number of deposits.
The problem with this sort of thing is that people say 'yes', then when the day comes closer they change their minds. As if it's too much trouble.
Oh, don't forget about the boring stuff too, like insurance. It shouldn't cost much, but if someone gets hurt at 'your' function they may possible sue. When arranging official functions in the army, we always had to get insurance.
You may also might like to write to the big and smaller window cleaning companies, when your numbers are finalised, and ask them if they'd donate 'gizits'; anything shiny would do. Or how about donating cash to off-set the cost for a bit of advertising on their behalf?