There is no way that I would charge any less than £20 for this job, more if it was leaded. Be confident and tell them your price and say that it includes frames and sills. If you don't try you don't get, if everybody accepts your price you are too cheap. At the end of the day a window cleaner on his own can only handle a certain amount of customers, so you might as well make them well paying customers.
If you price high and they think its too much you can always drop the price, but once you have given a low price it is difficult to put it up! and you may be surprised at how many will say yes!