Great - thanks very much for your helpful replies.
Final point(s) - no comment was made on putting down ply first. Presumably this is the right thing to do?
I guess I should allow an expansion gap around the edges of the room?
Yes they are narrow boards (well around 3-4 inches from memory).
So presumably this flexible adhesive is flexible enough to withstand being bashed about as the new boards are laid?
For a (sort of) barn conversion I'm working on in France, I've bought some lovely T&G merbau to go into the bathroom. This room is on the first floor and so had floorboards down. These boards were in quite a poor state so they've been treated and then covered with chipboard. The store in France...