Hi experts, Im having my bathroom tiled in a few weeks. When the tiler was here for the quote I mentioned that I intended to use the mapei wall tanking kit (see pic) before the tiles go on as the existing walls around the shower are dot and dab plasterboard with plaster skim finish. He said the tanking wasn’t really necessary, just an added expense that won’t make any difference as the adhesive and grout are both waterproof anyway, and that if anything it will be less reliable as the tiles will then only be stuck to the tanking, and not the wall itself. Any thoughts? I think after reading such good reviews on the product I am leaning towards using it anyway, belt and braces type thing. If I do, do I have to ask the tiler to make sure he uses Mapei branded adhesive to match the tanking? Or would any brand of adhesive be fine on top of the mapei tanking system? All advice appreciated.