All they really want is the price in writing on headed paper, see if you can download a quotation template off this forum some kind soul might send you one.
Headed paper with a sentence saying " quotation for window cleaning at big building, based on a once a month clean £5'000"
You can either keep it simple, or put together a brochure, I have tried both methods and both seem to work.
Sometimes its just that they need something in writing to cover there backs, ie if you verbally agree £20 then send in an invoice for £100 , at least they have it in writing.
Dave