Our downstairs rooms, notably the bedroom, have the benefit of internal double glazing to supplement the tower's windows. So effective are these, we actually have no need to use the underfloor heating in that room.
The main big room on the floor above is quite another matter. The tower's windows there are effectively radiators in reverse. They are cast iron frames with 4mm glass. Even with the underfloor heating now working properly the room does not heat up to a comfortable temperature and needs a boost from the flueless gas fire before it becomes usable on cold winter days.
We did our homework and decided the answer was internal shutters. To be effective these would need to be air-tight and of high insulation value. If set back slightly from the tower's original windows and with the intervening space trickle vented from the outside this should, in theory solve the winter condensation problem too.
Wonder of Wood today started fitting these into five windows and will resume tomorrow. Pictures then.
Just in time for Christmas.