I am modernising a 1930' semi and in bathroom which is tiled to half height have roved 2 layers of tiles to find a bed of mortar that is unstable so have taken this back to brick (all internal walls are brick). Plaster above is about 20mm thick and is clearly also not secure so logically will remove all to expose brick. Intend to ceramic tile from floor to ceiling throughout bathroom. Question is can I dot and dab with plaster board, will this be strong enough to hold weight of tiles, adhesive and grout as this seems to simplest option rather than fixing battens or wet plastering. Brickwork seems in fair condition and appears to be reasonably flat. Should I seal bricks with diluted PVA before adhering boards and should boards be sealed on external face before tiling? Around bath/shower screen area, is it necessary to use moisture proof board or can standard wall board be used with waterproof tile adhesive and waterproof grout?