Hmm - saw the thread title and thought maybe they were glazing over the whole of Sheffield to keep the noise in. Ah well... ;-)

If you're on more of a budget and your existing windows are OK, consider secondary glazing instead. It's actually better than double-glazing for sound insulation, and it's a damn sight cheaper too. It doesn't look as good though.

I get "Which?" and all the country-wide window-fitting franchises have had bad write-ups. Local places always seem to come out better for service/quality, and usually for cost too. I could let you know who did ours, but they're in Cambridgeshire so unlikely to be much use to you.

Graham.