Many thanks for all advice.

The conservation area police have now been unleashed! They seem to be
fairly keen, contrary to my original cynical view. They intend to pay a
visit today while the old sashes are still lying forlornly in the skip
outside the house, and the tupperware windows still have their sticky
labels on. Not sure whether they will go so far as to force a refit -
we'll see.

Cheers!

Martin Pentreath