I live down South and I would try for £20, if they don't like the price you can always drop the price a few pounds, mind you I always clean the frames and sills too. Why would you not clean them if they were leaded? With practice you can clean leaded just as quick if not quicker using a damp scrim followed by a dry one.
After saying that I priced up for a couple who have just moved from the Midlands recently, I priced their 3 bed house up at £15, they nearly fell over backwards and said that the cleaner they had in the Midlands used to charge them £4 for a similar property!!
they have no chance of getting someone that cheap down here. Price as high as you can, you will be surprised how much people are prepared to pay for a good reliable wc!