Any suggestions / ideas? Only thing I can think of is aluminium checker plate. Ceramic tiles obviously but this is on top of ply flooring.

Ceramic tiles, job done.

Could do cement board topped with vinyl (or similar). Vinyl for waterproofing, cement board for fire. Not sure thats any easier than tiling though.

Rick's cement board plus vinyl idea is sound, but whatever you finish it with I would tank the floor first and run the waterproof layer up the walls a little. If a cylinder or a valve lets go overnight, that keeps the water contained in the room instead of soaking into the ply floor underneath. Just make sure the seams and the door threshold are properly sealed, because that is where the water always finds a way out.