I am completely refitting my mum's tiny bathroom 1600mm x 2350mm. Sorted on how to do the plumbing and tiling (get a friend who has done several bathroom refits to help !!!). Not sure on the flooring though under the shower tray. As the floorboards are horrendous, was going to lay marine ply over the whole floor with round cut outs for the pipes and shower drain. Silcone the joints, countersunk stainless steel screws and pipes. Then lay the vinyl with holes for pipes and shower drain. Fit shower tray and the use clear silicone around the tray. Is this the correct way ?