Ok, so I have lifted the tiles and the board underneath was damp so waiting to dry out. The previous owners used this quite porous board which has swelled and caused the obvious cracking of the tiles as it pushed them up. What I need to know is what can I paint over this material to seal it when dry before I tile over ? I did think of the rubber paint you can get to paint on walls to stop damp coming through ? Then I will tile with the better adhesive and use flexible powder grout which will hopefully not crack and thus stop water getting onto the board. I know it's probably best to use ply board but that would be a huge job for me. Just for now I want to get the tiles back down and sealed.