The Natural Philosopher wrote:
...
How viable is underfloor heating in this situation?

Its definitely on, but not (as) cheap or easy (as in a screed).

snip

Thanks for your suggestions. Unfortunately building up a solid floor
brings problems here (although I can see the advantages too). I have
an under-croft beneath the conservatory which is then built on top a
block and beam platform. I don't fancy the loading of filling the
340mm space between "floor" and platform with hardcore etc. It was
designed to take a single story domestic load but not extra tons of
concrete on the platform! And I do like the suspended floor feel. But
it is good to know the pitfalls.

One doubt I have is whether any UFH will be able to generate
sufficient heat for the room. It was designed to maximise warmth, but
it is still a conservatory with lots of glass not a traditional
building. I am trying to do some basic heat loss calcs on what the
heat requirements could be but that is always speculative. Can anyone
tell me if they use UFH in a conservatory how warm it is?

Dave