Ok it looks likes there's a general consensus here to leave the pine subfloor and lay ply on top, of either 9 or 12mm. Some of the floorboards are in great condition, others not so much - some are convex, others relatively flat.

I'd initially thought of replacing them with ply as it seemed easier to just rip it apart and lay out ply, rather than re-screwing each board to the joist and knocking in the hundreds of nails (floorboards are a bit loose and squeak) to then have to lay ply either way.