As keith says on a concrete floor its best to use a layer of insulation below the mat. This can be stuck to the screed with flexible tile adhesive. I've used the
easy-heat insulation on the last few floors.
The sensor can be fitted into a groove cut in the insulation, best to fit it inside a small diameter conduit. I tend to use the
Devi mat which comes with the conduit
Lay the mat onto the insulation then fix your stone onto this with a white flexible abhesive as grey can bleed through the stone. Seal the stone once laid then grout with flexible grout and seal again.
Wiring should be done by Part P qualified sparks and the mat should have RCD protection. Test the resistance of the mat throughout the installation to ensure it does not get damaged and don't switch on for 28days. Again as Keith says treat it as a way to take the chill of the floor not the only means of heating the room, a heated towel rail off the hot water circuit to the cylinder is the best bet here
Your stud wall will need a flexible white adhesive, masonary walls either flexi or solid. What is the wall surface you will be pushing the weight limit for skimmed walls with 12mm stone which is a flooring thickness, OK if its 10mm.
Jason