I was trying to keep my post brief, but I feel I have raised too many questions doing this, so the full story is.
Site = bathroom floor (the drip area, if you pardon the expression).
Size approx. 2sq meters.
Builder screwed plywood down ready to tile but ran out of time, he said he would be back sometime. It's been a long, long time.
Rang another tiler, he has moved up north. Rang another tiler, job too small, but will do it for £***. Rang a builder friend, he will do it for me one day, been waiting a long, long time. Rang another tiler, he turned up. He went away, it's been a long, long time. In a way, this is good, it means there is plenty of work out there for good workmen. However, over a year on, I am going to, with the knowledge from this forum, do it myself. So I am where I am. I have the ingredients listed above in my original post. From the top of the plywood, I have 13mm depth to play with. The tiles will take up 5mm. I hope this helps you to help me.
From what I can understand so far is, I should take away the ply. I will do that right now. It might take me a while, because it is screwed down very solidly. I hope it isn't glued down too. The ply is about 5mm, when removed, this will give me a depth of 18mm to work with. While I am doing that, can you tell me my next stage thanks.
Thank you all so much for your interest.