Hi there hope I can be of some help.
See my post on Windows recently.
I got details of what to look for when buying windowswith regard to different types of double glazing and why some are more expensive than others. Its worth looking at it here is the link, at least you could fiund out why quotes are maybe a bit cheaper and compare quality.
Can t getlink to insert here but click on The SEI website with publication to do with insulating your home: Publications tab, "your home Publications" and then click "Guide to Insulating your home"
Hope it helps, It did for us anyway!