Cannot say definitely but sounds as if the actuator is buggered which unfortunately for you may mean major plumbing work to replace, I just replaced mine which fortunately was a Drayton 3 port valve which meant I could just simply replace by removing the complete actuator box off the valve by pressing a clip and rewire a new one in. Yours may be the same, try unclipping it off the valve.
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Just to give you a better understanding of how it all works I found this quite useful, if you have a neon screw driver or multi meter and are comfortable working with electrics you could check whether you are getting the necessary feeds to the actuator wires at the junction box when calling for hot water or heat etc Good luck
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