Tosh, you must have other very similar houses on your books, charge him the same.
If a pound a window is your usual rate, then that's what you charge, it's the same for everyone, do the job and bill him, your conscience is clear.
The other thing that has to be considered is the proximity to your other work. If you pass the house anyway, you can charge a bit less than if you have to make a special journey.
As was said above, you have the advantage of knowing exactly how long the job will take, and how long it takes to get there. £25 an hour is a very fair rate I.M.O, Dai
