I live in an old cottage which had uPVC double glazing done 20-odd years back. The windows need replacing and I'd like to go for wood. Since the plastic ones are standard sizes, is there somewhere I can get "off the peg" wooden replacements. Everywhere I've looked seems to want to make them to measure at huge expense. I live in the Bristol area. Many thanks for any advice.
Your windows will be imperial sizes which are now non-standard. Modifying catalog windows can be done but in most cases it will be difficult and a bodge.
Bite the bullet and buy hardwood bespoke windows.