Bona is always good. Also Wakol. What is the heating element? Electrical or water? If electrical DO NOT apply the glue directly over the heating element.

I think there are water pipes under the concrete floor. It is a 1950s buliding . After much research I've realised that gluing down the floor is best but here in France there are many shops which state that I'd need to glue a layer of cork under the wood. They say that for the neighbors and possible future legal conflict it's necessary. Is this the case or can I just glue the wood straight onto the concrete floor? Also, is it really not advisable to float the floor? The cost of extra adhesive and cork will be significant - not to mention the extra work load!

Who told you not to float???

You can float any floor over any substrate. And cork is a waste of time. The nice thing about is that you can lay it then sand out any humps easier then grinding concrete. But you should lay some sort of underlayment wether it be tar paper or something else.

If you want to reduce the amount of noise that can be cause from walking on the floor they have underlayments that are designed for such purposes. They are expensive ($200 for 100sq ft) but being concrete your not going to be disturbing your neighbors.

Your decision.