The message
from The Natural Philosopher contains these words:

Then get digging. For my money, putting in UFH with any less is a waste
of your time and your money.

I'd agree. I honestly think the whole idea of UHF without a really thick
layer of insulation is misguided. I've used 100mm poly under the
conservatory floor.

--
Skipweasel
Pay no attention to that man behind the curtain.