Roger wrote:
The message
from The Wanderer contains these words:

It has been suggested that I have a faulty motorised valve which is
stuck in the 'on' position.

That alone won't give rise to the symptoms you describe. If the system has
been wired properly, then the controller overides any other auxiliary
switches (like the thermostat, the aux switches on the valves) in the
circuit.

I don't think that follows. Surely the microswitch on the valve is fed
from the permanent live and the controller provides a switched live for
the thermostat. (That is certainly the way my controller should be
wired).

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


If the system worked properly once and the wiring has not been disturbed
a miss-wiring fault should not be present.

To the OP if the valve is difficult to move then that is the likely
culprit but it is the wet part that is at fault so you will have to
change the whole unit. The actuator can be tested easily enough by
seeing if the symptoms persist while the actuator is mechanically
disconnected to the valve which should, if faulty, remain open when the
actuator is disconnected. If the actuator is not faulty the thermostat
should then control the firing of the boiler.