boiler won't turn off

[cmiller]

Keston C25 boiler, Horstmann C27 controller
Checked all circuitry, the 'HW off' connection from the HW (terminal 1) seems to have a voltage on it, less than 240V but enough to light a neon, at all times, irrespective of external controls. (Whils the Horstmann is plugged in) This seems to be finding its way onto the Switched live to the boiler, switching it on. The only way to prevent the system 'doing it's own thing' is to switch off at the mains supply or remove the Horstmann.
I surmise that the motorised 3 port valve could be faulty allowing some 'back feeding of voltage'. Is this a common syndrome? All earthing and connectors are sound and I have tested their integrity electrically. I have tried a new Horstmann, no good.

Any Suggestions welcomed....

[htg engineer]

Has this always been a problem or has it just started ? any re-wiring carried out or any alteration to heating wiring ?

Worth changing the actuator, as they do cause some weird and wonderful faults.