In your humble opinions, which adhesive have you found the best to use over a ply floor with underfloor heating?? The normal two part flexi or am I missing something ??

Have you considered using a self levelling compound over the UFH?

Not enough info there Rockyroo. How thick is the ply, have you skimmed over with SLC, if so was it a latex slc?

Sorry grumpy, not even seen the job yet, phone call from a builder with the usual lack of information and then the moblie goes dead, but done work for him before and he uses 10-12mm plywood, I take it that you would use latex SLC on a wooden floor, and is it ok to tile straight on top of that, and how long should you wait

Yes I would run over with Latex based slc such as Arditex or anyone elses equivalent. I would then use a good flexible rapidset for the tiling or standard set if you prefer. As for how long to wait, that would depend on the slc you use. Arditex for instance should be tileable after about 4 hours but it depends how thick you put it down. leave overnight if possible.

If the ply isn't 15mm or thicker, techinically you should really use a 2 part addy to avoid warrenty issues, but it depends really how solid the original floor is. it would be your call.

without want to show you how to suck eggs. Remember to check the underfloor heating before you put the slc down, afterwards, before you tile and again afterwards. The earlier you spot a problem the less you will need to remove to sort it.