Another oddity (to me) is that rebuild costs seem to be a lot higher for
older houses than for modern ones. Why is that? If you're having to
rebuild from scratch in the event of total loss, what difference does it
make how old the original burnt out shell was?

Because if my house burnt down, I'd expect it to be rebuilt with the same
patterned brick and wooden sash windows that wouldn't be required rebuilding
some crap and cheap uPVC and stretcher bonded modern hutch. I'm also not
losing my intricate plaster mouldings that adorn the staircase. That all
costs, and I have no problem paying higher premiums to cover the risk.

Christian.