It happens that JoeJoe formulated :
Hold fire...

Just turned the boiler on again (it is not cold enough for the pipes to
freeze BTW), and I am certain that I heard the pump working!

All's well, I hope, for now.

Thanks again for taking the time.

I run a dehumidifier in the utility, to enable clothes to be dried
quickly and efficiently in there, without on making the place damp.
Rather than constantly monitoring the automaticly run dehumidifier, I
use a simple 'tell tale'. A clear plastic drain pipe runs down from the
dehumidifier, to a drain. I have that arranged so the pipe rises so as
to pass over an ordinary clothes peg, which causes the pipe to retain a
small bubble of water. At a glance, I can check the bubble of water is
there which confirms the dehumidifier is producing condensate and
working fine, even though it might not be running at the time I decide
to check it. Just a thought..