Lewis,
What service do you offer?
Just glass?
Glass and frames?
Glass, frames and sills.
Do you include wiping down ordinary doors and surrounds.
To get tip top prices, you will have to be different from others.
What you quoted for this one does sound a bit top heavy, even here in the rich south

£15 would be a good price here, may have been able to push £18, by including "all" as above on every clean and smiling a lot

A good tip is when you give a price and the custie thinks its a bit high, you can always back track and say well that is for the first clean only and after that every clean will be £.... ( take a third off for the future cleans).
Just think though what you would feel like if he had gone for £18.00......so keep going.

Why not clean your own (properly including all frames sills etc) don't rush, go at a sustainable speed and time yourself. This will give you a good idea of how long new jobs will take, so when you quote it will be easier to work out what the hourly rate will be.
Also while you are relatively slow, time yourself in the same way at custies houses. Each time you clean it will get faster and that will motivate you
