My parents had perenial windows too, and they did an excellent job.
they did the full house, it was a difficult job too, as my mother had old victorian windows full of horse hair to insulate them, they made good all the work that needed to be done and my Mum was delighted they were competitive with their prices too, and if you get a cheaper quote tell them, they will usualy match it.
I have worked for two double glazing firms so I am aware of what can go wrong, so definately go on recomendation rather than price.
Window frames come pretty standard, but if the fitter is bad, then you can have numerous problems, ask to see a portfolio of the company you go to of houses that they have done and go and see their work, if you see the home owner, then ask them how you think the company faired, most people are keen to tell you.
I know perenial (is that how its spelt) have a portfolio as my mother and I went to have a look at jobs done.
I had C.E.S windows in levenshulme to do my windows, and I think the fitter was called Jason, he did my friends, and thats why I asked them to do ours (this was ten years ago mind) and we have been delighted, they fitted our windows so well (full house) and we didnt even need to redecorate or replace wallpaper or plaster)
My parents only had their windows last year, so if I was to replace them again, I would definately go to perenial.
