"The Natural Philosopher" wrote

with copper, even a few mm of styrene or depron will make a huge
difference. And even with plastic barrier type pipe.

Since plastic in screed is what my UFH does, I can assure you its a pretty
poor way to transfer heat! Even with 100m of pipe the water going in at
50C is hard put to come out much less than 40C!!!

So I would not worry at all. Plastic in conduit sounds pretty good.

Thanks NP
I was thinking maybe I couldn't be bothered messing with the insulation -
see how the job goes tonight.
I suppose if the wall does get a bit warm, it's a good reminder that the
pipe's there for future.
Amazing how quickly you (or at least I) forget exactly where things are
routed etc.

Phil