Hi Pete, if you want to keep it simple just use a magic eraser pad damp with water and scrub the area to remove all traces of adhesive labels, just finish off with microfiber and job done.
Tadgh, have you ever done a builders clean?
In my experience, wetting the glass, brute strength along with strong fingers and a scraper is the only answer.
Pete, what ever price you are thinking of, and this may sound hard to say when quoting, triple it. If you are basing the price on first clean domestics, quadruple it or even higher.
I think you'll be surprised at how many builders will accept the price. Window cleaning costs is minute compared to their other costs even though it seems stupidly high to window cleaners.