Thanks for the replies so far. Tabby made a good point about the
variation in step height, which was enough to worry me.

Harry's point about wiring under the landing floor is understood but
on the rare occasions that I have had to access under the floor in
other rooms, I haven't found it to be especially onerous to unscrew
the ply. If anything the landing is easier as there tends to be less
furniture to get out of the way and it's smaller than any of the
rooms. However, if I go for Tabby's suggestion of using hardboard, as
this is usually fixed with nails (panel pins) as I understand, it
makes accessibility and re-instatement a bit more difficult.

Having done a bit more research I have a further suggestion. It seems
to be possible to get nosing covers in a variety of materials,
thicknesses and profiles. It occurs to me that if I could get
something suitable with a 6mm depth, I could affix these to all stairs
and put plywood on the treads as well as the landing. This keeps a
consistent rise for all stairs. Obviously this is a lot more work but
if I am prepared to do it, can anyone see any problems?

Thanks,

Bob