Cant use upvc but narrow gap sealed units are allowed, however some conservation officers say they arent, just like lots of planner officers they make it up as they go along5k grant to insulate homes ,,to replace the windows in this house it would twice that and more and thats if it was allowed ,,G2 listed ,not allowed to use pvc or sealed unit windows ,,must be wood and single glazed ,,the windows are painted up to stop the drafts .