I have dug Out the ground floor to be replaced with a timber floor and insulation, and with more head room. What do you think about doing away with tons of concrete/screed and using following construction; Undisturbed compact subsoil
Inch of sand dead level
3 or 4 inch pu insulation
18 mm shuttering ply
Tiling spline glued and screwed
Pine solid floorboards, no t and g, nailed with 60 mm floor brads
Basicly a 65mm thick solid floating floor.