Can I just add that the British Standard for Levelling/Wearing screeds for domestic applications where the screed is floating (as in this case on top of the separating layer & insulation) advises a minimum screed depth of 65mm.
It also mentions about crack reinforcement – usually a steel mesh placed in the middle third of the screed, however not practical with UFH so people sometimes go for a fibre reinforced screed. Just bear in mind the reinforcement isn’t to stop cracking but to limit the crack once it has occurred if that makes sense.
You could possibly omit the sand blinding layer if you put the DPM on top of the slab although the NHBC stipulates that in this instance it would need to be a paint applied product, something Like LAC by RIW.
Also if there is an existing DPM in the floor this may well be lapped with in the perimeter walls with the DPC – if possible try to get the builder to leave some of the DPC exposed form the wall so you can link your new DPM with it (this might seem a built belt and braces but that’s what I’m like)
Also with underfloor heating it is best to have a thermally neutral floor covering tiles/carpet etc. If real wood, it may act as an insulator against the UFH meaning the system has to work harder. If you google it there are arguments for and against this principle.
Obviously all the above is hypothetical and in no way to be construed as though I know what I’m talking about in any professional capacity.
EDIT: of the ninja variety