£75 an hour? You'll be the cheapest by far, you'll need to double that to make any serious money after overheads.
Re builders cleans, yes always best to quote an hourly rate as they can take longer, anywhere around £75/hr should secure the job. Just ask that they don't show the joiners / plumbers / plasterers / scaffolders / brickies / tilers your quote, not too sure they will be pleased that the window cleaner is earning 4 x what they are.