I agree with Dave Polshaw. When our extension was being done, we asked the builder to recommend someone for the windows which needed doing on the rest of the house to match the extension. He quoted us himself, and suggested we also get two window firms to quote. His was so reasonable by comparison we had two new doors out of it!

Always use local tradespeople if you can, it keeps them in business, they'll be handy for problems or remedial work and they live by their reputation. The big companies promise the Earth with no intention of fulfilling any of the hype.