You would be surprised at how much some people are prepared to pay and how much they value any work done by a tradesman. These are the customers to actively seek out and by the sounds of it you are doing just that. A higher minimum is no bad thing, lets face it if a tradesman turns up to your house for a one-off job they will have a minimum price and most wouldn't bat an eye at £25 for the satellite repair man to fix the dish for his 10 min work. That's the way you should pitch your window cleaning. Esp as you wont have a run of a dozen houses next to each other.
There are also allot who feel it's "like glorified begging" and £4 is the max they will pay! Avoid these like the plague.
I also think that the larger your van the more you can charge! 
the difference between a 'tradesman' and a windy is that most tradesman have 4 years of training behind them, you wouldnt expect a plumber to turn up and then ask him how he learnt his job and he said 'oh i saw someone doing it and then read the net a bit about it and thought i can do that'!!!!
Which is why I feel qualified to comment as I have 20 years experience. It is also why I am happy to classify myself as a tradesman.
If we get it right in our own head we can come across better to the customer.