I have installed a new concrete slab in my bathroom and celotex insulation with underfloor piped heating clipped to this so am ready to now screed it. This will be 75mm thick which will then be stone tiled. Question is how/what should i mix to make the screed. I understnad that for underfloor heating a wet screed should be used so that it flows around/under the pipe. Can anone advise please.