I am doing my shower room with tiles 250x350. The walls are plastered. Not always dead smooth but not too bad. I have done the tiling around most of the room using tub ready mixed waterproof adhesive. Seems okay. I am going to do the shower cubicle area now. One wall tiling on top of plaster, some of it newly plastered and one wall onto existing tiles. Firstly, how long do I have to wait for new plaster to dry before tiling on it? Second, do I need to paint a special bond onto the plasted walls to make it really water resistant and to make the tiles stick really well, if so what do I get, and do I use the same stuff on top of the existing tiles, to make the new tiles stick?

A normal plastered wall will require approximately 1 month to dry before tiling. You are pushing the limits of tubbed adhesive with the size of tile you are fixing, it will take longer to dry out! Also there is no such thing as "waterproof" ready mixed adhesive. It will be water resistant and this DOES NOT mean water will not penetrate through, it means that the adhesive will not break down when it comes into contact with moisture. If you are tiling to plaster you will, for best protection, need "tank" the area you wish to tile.

Your next question, I am sure, is what is tanking? drop the word "tanking" in the search box at the top and it will bring up many hours of reading for you.

Finally, it is not the best idea to tile over existing tiles, much better to remove them and make good the walls then tile onto your now bare surface.

Oh goodness, we are doing everything wrong then. I am worried now. Thanks for your advice though.