andyl3004 wrote on Mar 21st, 2018, 6:47pm:Thanks all. Two follow up questions. I thought foil backed insulation reacted with some Portland cements so should be avoided?
You would need a damp proof membrane between the concrete and the insulation so there would be no contact, but I've just looked on the Celotex website and they are not recommending using it under a slab so you are right it's better to find a different insulation material, but you may have problems finding a suitable one thin enough.
For .1 cubic metres you will only need a couple of bags of cement but I wouldn't use the plastering sand it's likely to crack. Best to use all sharp sand.