I used these and was happy with the service. Owned by a guy called Wayne and not our star striker.
I'd get three quotes and make sure they are pricing like for like, e.g. 'A' grade units. From memory I think I paid 5k for 11 'A' grade windows and composite front and back doors. They did my conservatory too, so can't remember the exact breakdown. From the quotes I had there was about a 8k difference between the three for the whole job!
http://www.ricowindows-ltd.co.uk/