We had our Victorian sash windows replaced with new double-glazed wooden sashes about two years ago. I had them done by a man based in East Dulwich. He makes the frames, has them glazed, then installs them with the help of a friend. Custom-made windows at a fraction of the price the big companies were quoting (some of them quoted double what I ended up paying) He also refurbished some windows that were still sound but needed new cords and draught proofing. His name is Guy Chadwick (The Sash Guy) and you can find more recommendations for him and his number on the East Dulwich Forum. I couldn't tell you what his after sales service is like because we haven't had a problem with any of the windows since!

I've heard horror stories about the big firms and would steer clear.