Antony wrote:
We are renovating 2 attic rooms - stripping and varnishing doors and
windows, wallpapering and sanding the floorboards.

At what stage should we sand the floor?

Before you wallpaper them.

Seriously though.
Sanding will remove nearly all marks, but paint that's been left to soak
in may be tough.

Personally, I'd do the sanding of the floorboards almost last, as you've
then got less chance to damage the surface.