bill wrote:

All very helpful, thanks, so how about Plan B, I take up the existing
tiles, and lay underfloor heating? If I do, does that have to have some form
of insulation board underneath the cable, or doesn't it make a lot of
difference?


You SHOULD have about 4" of screed over about 2" of polystyrene
insulation to get decent efficiency out of the UFH, otherwise about half
the heat will be going to making the worms cosy, rather than your cat...

SO, if you have 6" to spare, or can rip up the floor to that depth, go
for it.