The message .com
from "Nigel Frost" contains these words:

I'm wondering how to enable the UFH to be kept on (perhaps timed, or
perhaps just thermostat) independently of the main heating
thermostat/controller.

You may well find you don't need to. The thermal mass of 3" of concrete
will keep it warm for quite a considerable time after the heating's gone
off.

The system I've installed (but not yet connected) in the conservatory
has a thermostat in the return path. That in effect sets the temperature
of the floor.

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