I wished I'd had the guts to have a £10 'minimum charge', instead as some of you have pointed out, when you get started you tend to get a bit desperate and instinctively lower your prices.
Having said that, I've got some work that I canvassed where the most expensive house in the close is £7.00. However, because the work is so compact I can easily make £25 per hour.
This is one thing to bear in mind when canvassing new work. If you know that the area needs a window cleaner, it may be more profitable to drop your charge SLIGHTLY, and sign up a higher number of customers, rather than getting better 'Money Per House' but less houses overall.
Pricing up new work seems a bit of a dark art to me, you've just got to look at the overall returns from a patch of work and figure out what you can afford to charge versus what someone from that area is likely to pay