Thanks :) Yeah decided to go with tile. So sand cement is ok over existing tiles if I prep them first? Only problem is, the new pad can only be about 30mm at it's thickest (leaving up to 15mm for tiles) otherwise it'll end up being higher than the hallway floor. So I either need to make the fall shallower or not bother with the waste and make it all level. If I made it level, could I use levelling compound at that thickness or would deck mud/sand cement be the best option? Also, should I be putting in mesh for strength? (I know I should (and will) ask the local hardware what they suggest, but it's great to have suggestions from the Houzz community also)

If the flooring guy doesn't have any suggestions, he's not a flooring professional. Fire him and hire a real contractor who can deal with subsidence and settling issues.

Bump

We have chosen Torlys Laminate now. It is more expensive but more resistant to water and small elderly dogs. The flooring isn't down yet as the house is still under construction. We await with anticipation on the outcome of our trial of the Green Glue noise reducing tape, in conjunction with between floor insulation and well laid and attached particle board.