If you're short of work:
1. Knock on doors.
2. Leaflet.
3. Put adverts up in shops.
4. Ask current customers to recommend you to their friends and family.
5. Sign written vans/uniforms can't hurt.
6. Advertise verbally to your current customers about gutter cleaning, or whatever add ons you do.
But a blanket £10 for all doesn't sound good to me. In any area there's a range of houses from big to small; from privately owned to social accommodation. It's purposely planned to be like this, so you cannot possibly gauge an area and say they're all £10 houses. Some will have access problem, some will have conservatories, some will have extensions.
The type of customers we are after are people who have a few spare quid, who are willing to pay a fair price, REGULARLY, for a regular window cleaning service.
And our business is extremely labour intensive. We can only clean a finite number of properties in any given day, unlike shops or pizza places who can sell (within reason) an 'infinite' number of products in one day. Therefore we should be aiming to get the best customers and prices we can for our efforts.