I have googled but cannot find an online window supply company that gives
guide prices etc for supply only. Looking to replace 4 twenty year old
double glazed PVCU windows with similar. Any sites you can recommend?

Premdor do wooden framed double glazed casements, which are an improvement
on manky uPVC. Guide prices are online. You can expect about a 25-30%
discount when you purchase, probably through a builder's merchant.

http://www.premdoruk.com/

Typical example: 1200mm wide, 1200mm high side hung, two opening light with
one internal spacer horizontal bar low-E glass is 380GBP before discount and
VAT, or about 312GBP.

Christian.