Owning your own home

my neighbour opp has timber, I have UPVC, its hard to tell

I refurbish sash windows, retaining as much of the originals as possible. Sometimes changing all the glass for 6.3 acoustic, upgrading weights, sealant instead of putty, adding thorough draftproofing. It's a 4 week job per window for me. Size does matter but £2-2.5k is not unreasonable. The problem with replacement windows in comparison is the damage to the interior/exterior decoration that needs repairing. Fitting is usually the weak link with replacements.

This is not an advert, I have a full order book but there will be people all over who specialise in local windows.

I've done some really early grade II listed sashes, they were out of true in every dimension.

Have you spoken with conservation area and looked at the details of the conservation area? If it already has pvc then there’s a reasonable argument you should be able to replace with same material/sash style (if you wanted)