Thanks sean,definitely not using ply in those circumstances again,due to your explanation of wood,most builders i work for seem to always get ply,i will be advising against in future,its going to be either hardie backer,wedi or similar product or plasterboard [primed],leaving access for a plumber if ever they get a leak,never used those magnets,always used mirror screws,like the idea that you don't see any fixings.thanks again for all your input.