I'm going to have to choose some windows, and a window fitter soon.
Most of the windows that we want to reduce are in solid brickwork, so at least I can do the new brickwork myself. I like that idea, because it will allow me to add chamfered jambs, which will let in more light, and be an interesting feature.
Fitting windows to cavity walls is complicated, so I don't fancy doing it myself. But do I go to a local window company and get UPVC (U-value about 1.4) or to someone like Green Steps or Green Building Store who make wood-framed windows with a U-value of 0.8-0.9? I calculate the better insulation will save about 400W, which is about 5% of my current heating estimate. That's not a lot really, but they will also look better - and that is important.
Furthermore, who will fit the windows for me? Green Building Store offer a supply & fit service, but Green Steps seem to be supply only. I suspect that it'll be way cheaper to find a local fitter.