On 2019-01-02 11:59:25 +0000, David said:

Gave up on the diagnostics and changed the valve.

Quick and simple job of replacing wires like for like in the wiring loom.

It would have been easier if the wiring loom had been more accessible.

All now working as it should be, thank $Deity.

The original valve head has been battered for some reason; will post
pictures soon (if anyone cares).

You mean you just swapped out the motorised valve head and not the
valve itself?

They motorised parts certainly do fail - I've had to change both the HW
and CH valve actuators last year as they were getting stuck and
buzzing. Not worth refurbing the gears or changing motors. One is a
Honeywell the other a Danfoss, installed about 15 years ago when we
moved in and redid the system.

And yes, our wiring loom is under the stairs and hard to get at...