The gap from the wall to the tap end of the Bath

Hi Skyebirch, If I am understanding your question correctly you wish to transfer the gap from one end of the bath (the tap end) to the other (the foot end) if so you will have to build a new frame and this would be easiest done with wooden battens 3x2 or 4x2 should be perfect ... you will need to cut the batons to size so measure from the floor to the underside of the bath edge. Build your frame to "underside" of the bath, but make sure you leave enough wood sticking out from underneath the bath edge so you have room to secure a "waterproof tile backerboard" these boards are also called "insulation Boards" or "Construction Boards" and can be bought at Toops Tiles or any of the other tile centres, they are lightweight, simple to cut with just a Stanley knife, are 100% waterproof and are a perfect substrate for a tile. They also come in many different thicknesses so pick one that best suits your job.... Remember that you want the board to finish roughly 10mm below the bath "top" so that when you do fix the tiles on top they now finish flush with the bath, ideally you want the tile edge that is going to be against the bath about 1-2mm above the bath and the opposite edge should be 3-4mm higher, this will create a very slight slope in the tile that will hardly be visible and will act as a drain away system for any water that splashes onto your tiled surface, like I say leave the tiles ever so slightly above the bath and this will also prevent any water from "pooling". The edge of the tile that touches the bath should also be cut 1-2mm back from the bath, leaving a small gap between the two areas so you can run a silicone seal along this gap to make it totally watertight, it will also prevent any movement from when you are using the bath disturbing your tiles and eventually working them loose. There is no real need to treat the wood going under the bath however I would always prime them with an acrylic primer such as BAL's APD or SBR. Just as a point of reference for you, never never use PVA when it comes to tiling, it is not a suitable product and anybody who tells you otherwise is not a "professional" tiler. Using PVA to prime an area that is then going to be tiled runs a far higher risk of failing than using an acrylic primer designed especially for tile installation, one of the many reasons for this is PVA livens back up when it comes into contact with moisture (a bit like wall paper paste, you cant feel it on the surface but once its wet it becomes sticky and slimy) you do not want this reaction going on behind your tiles. Sorry for digressing slightly but its so important. Ok I don't think I have missed anything out however I am a professional tiler/bathroom fitter and not a professional writer :) Please feel free to ask any more questions. Regards Toby