Really struggling to pick company for UPVC windows, vast differences in quotes

[vez123]

So we are looking to replace all the windows in our new house and have decided on Rehau windows. We have been getting quotes in for our 3 bedroom 1930s house but they have been ranging from £7500 to £13000 for the same product! We are tempted to go with the cheap quote. We visited their show room, they are official Rehau suppliers, they showed us pictures of work they have done, all seemed good. We just don't understand why there are such big differences in price! Is there anything we should look out for? Would we take a huge risk by going for the cheap one? Can anyone recommend a reasonable but good quality window fitter in London?
Thanks!

When we had our windows changed this time we used someone who had changed the windows in our previous house. We now live in London and he travelled from Essex.

We also had Rehau, could the difference in price be because of the ratings? When we got quotes for our first house there was £2000 difference. The company we went with was home based so had no huge overheads, we were able to make sure they were FENSA registered and offered insurance backed guarantees. Even though we weren't able to go to a showroom I saw samples and pictures of previous installations.

When we got quotes for our house in London they were a whopping £3500 cheaper and the other company didnt quote using Rehau as requested!!

Where in London are you? I'm not sure if I should put a link to their website on here or if they will even come out to you but it's worth giving them a ring. They are called Draft seal doors and windows.

Thanks to you both for your replies.
Grograg, the quote is for identical windows - we gave each company the same spec.
Oldbag, we were also wondering if the companies just have different overheads. The cheaper ones are all based in a not so swanky area whilst one of the more expensive ones is based in an expensive area of South London. If this is the only difference then we would go for the cheaper ones. We will speak to them to justify their low and their high price respectively.
Such a difficult decision as it's such an important part of the house which we do not want to get wrong!