The message
from The Natural Philosopher contains these words:

It's wired from permanent live on the switched fused spur that should
run the whole heating system..

How come? The microswitch only closes when the thermostat powers up the
actuator.

Correct. Your point being?

Sorry about that. I thought you were saying that wiring the microswitch
that way would result in the heating being permanently on.

--
Roger Chapman