Have read all the other posts on here about tiling over a range of other floor surfaces (including laminate!) but can't find any reference to cork floor tiles. These are in an en-suite so it's a small area, they are quite thin, are well stuck down and have been sealed. I was thinking of raking the surface to give a good bond for the adhesive, but do the same warnings as given elsewhere apply to cork? If so, could I just overlay the cork tiles with ply and tile onto that? The sub-floor is flooring grade chipboard so it would hold a screw. Anything rather than have to try and get the tiles up!!!
Never tiled over cork or ever seen it mentioned. However, I don't think it is really of any significance.
You shouldn't tile straight on to the floor anyway, you must ply the floor first to remove any flex and create a separate layer not only so the existing floor can expand without damaging the tiles, but it can easily be removed in the future.
I don't see any reason not to ply over the existing tiles providing they are firm. Just use a screw long enough to penetrate the floor but not so long as it damages pipes below. (Been there, done it!)