The Initial Search

In anticipation of completing the purchase of our house in May 2012, we began research on looking at upgrading the windows. We are attempting to renovate the house to improve its layout and energy efficiency, so we started reading through forums such as http://www.greenbuildingforum.co.uk for advice and reviews. We came across the following useful 2009 post by "Maren": http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=4409

We have now received all but one of our window quotes. 
As we are already behind schedule and most of these 
suppliers have an 8-10 week lead time to delivery I 
need to decide on the supplier asap and order by the 
end of the week if poss. Any help or advice, any 
supplier to be avoided or recommended - all information 
is very welcome, I really need help on this. 

[...]

Quotes received:

Adpol (windows from Poland): 
£7727 inc VAT&Deliv; 8-9 weeks

ecomerchant (windows from Germany): 
£6861 inc VAT&Deliv; 4-5 weeks

Internorm (windows from Austria): 
£8684 inc VAT&Deliv, inc installation; 8-9 weeks

Nordan (windows from Norway): 
still waiting; 8-10 weeks

Rawington (windows from Germany): 
£8058 inc VAT&Deliv, inc installation; 8-9 weeks

Russell Timbertechnology (windows from Glasgow): 
£7737 inc VAT&Deliv; 7-8 weeks

We reviewed and contacted most of these vendors.