I was thinking of screwing some 3" x 2" timber around the walls, and then
fixing some battens to the timber at 600mm.
Does this sound reasonable ?

Should work fine. Make sure your "battens" (actually joists) are large
enough for the span. They should be "on edge", with the longer dimension
vertical. 600mm centres require 12.5mm plasterboard. You should only use
12.5mm plasterboard for habitable properties anyway, for fire resistance.

Christian.