Nanda

Hi, I have recently taken a house and trying to get wet underfloor heating installed (combi boiler based). I have very less knowledge of the building methods or construction methods so please excuse my ignorance with regards to the question. The house currently has laminate flooring with concrete layer underneath. I'm trying to get porcelain tile based flooring with wet underflooring. My builder has started off the process. The laminate flooring has been taken off now. And he has almost finished drilling off the floor. I can see gravel flooring now. As per my builder this is going to happen next....1) Preparatory work: Remove all the rubble, level off the current uneven gravel floor by taking off the sharp items. 2) Place insulation boards all over the flooring (50mm thick) 3) Then place the hot water piping, etc. 4) And then the concrete, screed and placing tiles. So I have just got a few questions with regards to steps 1 and 2 - I have been seen / read at multiple sources that a screed / concrete layer needs to go in first even before the insulation boards are fixed. This is probably to hold the boards secure(?). So, I'm now a little worried if there is a flaw with what my builder is doing. So, just wanted to see if my concerns are valid or not. Please help. Thanks