jacob wrote:
If I put a 800x800 shower tray in a 1200x800 space (on ply base over a
t&g floor) what is best for bringing up the level of the extra 600mm
floor to the rim of the tray, ready to tile?

Not with you - where does the 600mm come from?
Is the ply base sitting directly on the floorboards, or on a plinth
(usually needed to gain access to waste pipes, trap etc)
Are you wanting to raise the whole floor of the shower-room so that you
step *down* into the shower tray, is that it?

David