Faulty 2 Port Zone Valve ?

[DorsetDasher]

I think I have a problem with the CH 2 port zone valve. It works OK in the morning sequence but doesn't work in the afternoon and can only be `started' by turning the hot water off and on briefly . (Bizarre) . The `faulty' valve that controls the CH zone is an old Tower Heating Components Valve but the head is not available and the new interchangeable heads will not fit so in order to change the valve I will need to drain down etc. Is there any else I should be checking as it seems odd to me that it works as intended every morning but it won't work in the evening . could the valve be OK and should I be checking something else such as a thermostat or perhaps the programmer. This appears to be a Drayton LP711 or similar badged up as British Gas?
Any thoughts appreciated !

 

[MANDATE]

my initial thought is maybe it's not being supplied with power PM, due to faulty timer. Suppose you could test for power easily