AlexW wrote:
1) What's an appropriate thickness for the board, I want it to be thick
enough to disguise the board imperfections (under constant footfall) but
not so thick to loose too much of the skirting (which I'd rather not
refit) and introduce significant level differences with the landing.

3/16", with the outer grain parallel to the existing boards.


2) I have heard talk of wetting down the boards to get them to shrink a
little following fitting ... does this work / is this necessary, as
plywood has long grain running both ways which might the effect minimal?

Don't.


3) Are expansion gaps necessary around the edge (again the plywood
should be reasonably dimensionally stable)? If so what is a suitable size?

Tiny, 1/8" or 3mm. What are you covering the gaps with?


Any other tips/info would be greatly appreciated.

I have wondered about using 1/8" (3mm) cork sheets in 2'x3'
(600mmx900mm) size for a cork floor. Unfortunately, these are about
£3.50 a go. You might need about 10 sheets. It might look nicer than the
"normal" size.