BMD,
We are putting 40mm insulation on the first floor, between the screed and the precast units.
I spoke to some people who said we are wrong to do this, including the owner / director of a regional construction firm, who has UFH. Their opinion was that with UFH you need to think about heating the whole house and that the heat in the precast units will be transferred to the rest of the house.
One flaw with this view, I think, is that you are expecting the heat to travel downwards through your plasterboard on the ceiling, as well as through the cavity between the battens and the plasterboard - 100mm or more if you have HRV ducts in this cavity.
UFH installer was in favour of the 40mm insulation and his points seemed to make sense.
We'll see I guess, good luck with it.
boots