The most excellent ganching commented on the last post:
The old windows were this really strange type of double glazing, there was quite a big gap between the panes and they were sealed-not-sealed. This meant you couldn’t access for cleaning, but also meant that *stuff* like bugs could get inside, and exciting cultures of mould and mildew could take hold. The opening itself was quite tiny, so impossible to access the outside for cleaning (it’s quite high up).
I hope the improved water-tightness is as spectacular as the improved view.