Julian
If I was doing the quote I would compile a dossier which includes, a Risk Assessment form, a Method Statement, a Copy of insurance, a Environment Policy and a Copy of The Working at Heights 2005 Directive.
I would point out the risks involved in the cleaning of their building and how you intend to do so safely, which is cleaning with wfp, because of the 3rd floor work.
You say their are 2 of you working, so this job should be completed in 1 hour 15 minutes maximum,and 2 hours 30 minutes if you were working on your own. (excluding traveling time)
As for the price, is it going out to tender or are you the only one who has been asked.
If you are the only bidder then I would quote high and reduce the price in discussions with them if you need to.
If its going out to tender, then beware, there are some companies now quoting as low as 35p per window, so this job could attract a price of £50.00, but I would say a figure of £85.00 to £100.00 would be more likely.
The most important thing to remember and do is, don't quote to low just to get the job, you will soon regret it and come to hate doing it.