Thanks very much for replies so far.

I like the idea of making the doors up from scratch.
However, I'd be concerned about warping with the
"big sheet of plywood" approach, so I'd probably
prefer doing it properly. Hmmmm. I must admit
that time is a real issue for me at the moment
too, so the flat door + mouldings approach is
still looking attractive, even though the end
result would be less than ideal.

I won't be taking the door frames out - the
101-year-old original architraves, skirtings etc.
are the best things about my flat.

Someone asked what was so wrong with the present doors -
imagine the worst frosted-glass, cheapo early 70's
efforts in shocking condition and you're there.
Every fibre of my being (and I've put on weight recently)
is offended at the sight of them.