If you have I'd love to know which firm you used, what type of glazing you installed, and if you were happy with the windows and the company you used.
Our flat is in a listed building in a conservation area, so I know the regulations about what we can do will be very strict. Definitely no PVC, and I think that the Council prefer you to refurbish original windows rather than replace.
None of the neighbours have had double glazing installed, so can't get any personal recommendations. All the websites I've been looking at seem very professional and have great marketing, but I'd put far more value on hearing from a happy client. Hoping that someone can offer some advice........