Simon wrote:

Hmm so it seems the concensus is that the fibre cement is the better
material, so I'll go with that approach. Is it actually "better" than
the old asbestos stuff, or could my original one just have been badly
installed? I say that because the last roof is probably only 10-15
years old, way less than the 40 years lifetime suggested for fibre
cement.

asbestos sheet _is_ fibre reinforced cement, the fibre used was
asbestos. New fibre cement boarsd simply uses other fibres, eg nylon,
polypropylene, glass.

NT