I drop a cloth over the brush when they've stopped running with water and wipe down the sill. I was gonna start a thread to ask about this as it takes a fair bit of time...interested to hear what the established pro's think.
Dont worry bout it then. Cant be doing with people like that. If they had of asked you to pop back and sort it fair enough, but they dropped you just like that. I had exaclty the same the other day on a 3rd story, as jo said they should be happy enough to get their windows cleaned!! Not to worry about the sills. He told me hes "paying you this time but i dont want you to do them again and im going to find a way to do it myself" ... replied with "fair enough, bye" I offered to sort it for him and that wasnt good enough so you cant win with people like this. Dont worry yourself about it and write it off.