Tosh,
I don't mind quarterly cleans if the price is right, but I know what you mean regarding cleaning them in the 1st place. The 2 schools (2 mins and 5 mins from my house), that my 2 of my 3 children go to, both their windows are filthy, and this one has taken months to get back to me, after the wife spoke to the head mistress and I sent a follow up letter offering a quote, so I'm not getting my hopes, and like you will just offer an honest quote and see what happens.
Lee