[widepants]

right heres a quick plan of what you need to do and things to think about.
The loo looks like its been there for a while so now is the time to put in a new one , if you are happy with it then take it out and refit on new tiles.
Figure out if you can get an eletrical supply to room to run heating mat.If you can then follow next plan.
Strip up the old floor and seal it , you could smooth it out now if needed but not that important at the moment.
Fix insulated board using a cement based flexible adhesive with a floor grade trowel.
Fix under floor heating,a loose wire system will be better in that room.
Skim over with a flexible latex leveling compound
fix tiles with a flexible cement based adhesive (chose colour according to tiles)
Grout with flexible grout.
refit loo (of course dont fit heating under the loo)
after floor set , turn the heating on low and gradually bring up temp for the first time.
stand back and hopefully admire