The hourly rate means nothing, it is dependant on the speed you work at and we all work at differing speeds.
The best thing for you, as a new starter, is price per window.
3 bed house - 10 windows @ 75p, 2 glass doors @ 50p = £8.50. If you can get enough of them and crack off 2.5 - 3 per hour then you're off the mark.
There are people out there doing if for half that price, others doing it for double, you just have to work out what your market will withstand, without you been seen as too greedy or ridiculously cheap.
I had a woman just yesterday on the phone asking for a price for her windows, 3 bed semi, I told her it would be around £8 -£10, there was a pause then she said she would "get back to me", however as she was hanging up I still had the phone to my ear and I heard her say "Jesus Christ £10!!!"
Guess I was too expensive for her then.