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The guy I used charged £300 (plus 5% vat) for refurb or £1,100 (£800 at 20% vat and £300 at 5% vat) for new hardwood double glazed (the quality of the hardwood ones was way above some others I had installed).
At some point I'll probably try and work out the impact (my thermostat has loads of data to download but I got a loft conversion at the same time which complicates things).
Having windows that smoothly open and close is a big bonus though.
Ventrolla will replace shagged out old sashes with new double glazed wood ones and repair the boxes for ~£1k per window (with decent discounts if you have many windows).