I have a 'clearance issue' with the floor of my barn conversion in that I don't have much room to add in insulation between the current floor (which can't be dug out for structural reasons) and the floating wooden floor that's meant to go on top of it. I only have about 50mm. Can anyone recommend a really efficient thin foam floor insulation - or any other material for that matter?

There's a product called Thinsulate that gets used in boats - 25mm thickness I think, and made of something like polyester fleece. Probably a very expensive way to go, though.

I would definetly go the metal foil route, either bubblewrap or polyurathane variety. however, the bubblewrap version will introduce a spongeyness to the floor (albiet to a very small level). I have had similar issues to yours (Mill conversion - 75mm clearence) and used multiple layers of foil with hard poly foam to 1" and it does the job.

Its unlikely that they will meet building regulation due to the way they are tested (The hot box test) the only BBA certified multi foil insulations are TLX formally thinsulex and Kontrol but I think these are only for use in roof and require somewhere in the region of 70mm of a Celotex type product to achive the UVALUE. Some building control bodies are accepting other ones such as Tri iso 10, but more for an easy life rather than the fact that they work as they are meant to