Motorized Valve problem on Central Heating

Can anyone advise?
I have what I suspect is a faulty switch on a 20year old, ACL motorized valve on my central heating system. The valve is labled Model A679B308 3OU1.
The actuator has a "tin" shroud and on removing that, it appears to be held to the valve by two brass nuts with sealant or something similar on the ends of the threads, not very accessable either.
Does anyone know if the actuator on this type of valve is replaceable or is it a whole new replacement jobby.
Thanx. Keith.
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