hi mike,
£12.00 is about right for a house this size in swindon. Especially when you take into account the fact that you may need to use a pole on the window above the garage, and also charging a bit more for the bay window at the front.
As for the price being high due to the house looking like its in a well off area i tend to make the prices up as i go along but i try to suss out the owners and charge accordingly.
As for time to do the job i am fairly new to the craft but i would say 30 mins comfortably to do the job trad.
hope this helps,
rich