I've looked into the Zero rated VAT issue and it turns out that only alterations to a building are zero rated, not repairs. Furthermore in the recent budget announcement zero rating for listed buildings was scrapped altogether. We certainly don't want to install uPVC under any circumstances. We want timber sash in the exact style of the original but double glazed. I'm still unclear as to whether this will require planning permission. I will give the planning department a call later. Thanks for all replies