Thanks Jonathan. I was wondering about that. Our architect specified Celotex in the building control plans so I was investigating that, or similar products. I'm not sure if that decision was due to fire retardance or to other issues. Because I'm doing most of the project management it's important to know the order in which everything needs to be done, hence the question about what can be done above or below floorboards - it really does affect a lot of my decisions. I will look into foam as well as Celotex.

@Victoria - Many thanks , Can you please tell me if the special board was put on top of the floorboards or on top of the joists ? @Nathan Oliver I have heard about Omnie TorFloor and seen a few videos as well. Do you know how much they might cost per Sqm ?

When looking for my house which was very similar (concrete in one room, suspended in the other), I’ve been recommended once I have insulted between the joists, to install a structural chipboard floor with precut grooves (ThermRite Pro is what our supplier suggests) for the UFH and then your finished floor goes on top I’d intended to put oak boards on top of both as I already have them but everything online that I’ve read advises against using solid boards and instead to buy engineered boards. That seems like a big extra expense but one to consider.

We had our bedroom floor done 3 years, it’s about 27sqm. It only cost c1k but we didn’t need sound insulation. The guy was an antique furniture restorer who started refurbishing floors as a sideline and i suspect he seriously undercharged us..We also didn’t need much in terms of repair work, all boards were sound and only 1 had been replaced at some point in its lifetime, you may also find holes where central heating has been installed or gaps under skirting etc i think you need to consider whether you’d be happy with all the dents as pine is such a soft wood. We hung new curtains days after the floor being done and the ladder (sat on a towel) created dents. We put furniture pads under everything but have had no need to move the furniture since but i suspect it has big dents everywhere! It's fine for us jn a bedroom but not sure if i could cope in a living space