I accept that if I had 50mm of insulation it would make a difference,
but this is a basement with finished headroom of 6'6" so I don't have
space to play with.

Personally, I'm not sure you really have space to install it as is. You
definitely need the 6mm, and you really need the 50mm. Would it be possible
to dig out the floor and reinstall something more suitable with DPM (you
don't mention one) and 50mm of kingspan? You could even install wet
underfloor, then, which will be much more efficient.

Remember that because the heating is buried in the floor, you get direct
conduction of that expensive and environmentally unfriendly electric heat
right into the ground, so the u-Value is much more significant than a
standard floor, which is unlikely to be excessively problematic even
uninsulated.

Christian.