Help... intermittent hot water issue

[SJB060685]

I would pop down to your local screwfix or toolstation and get a cheap multimeter, as i said it doesn't need to be expensive, you'd probably get one for around £15-£20.
From there I would be looking for 230 volts between a neutral and the white wire on the actuator head. What that tells you will allow you to go from there.

 

[Aquarius]

Does it need batteries in the thermostat? Have you checked the batteries, and are they alkaline?

 

[Larches]

Does it need batteries in the thermostat? Have you checked the batteries, and are they alkaline?

Yes, they're Energizer alkalines, replaced not that long ago. I think the thermostat is working fine - I can hear it click on (and the flame symbol appears) when I notch up the set temperature, and as mentioned, the actuator motor responds with this slight buzzing, and then stops when the thermostat clicks off again. As per @SJB060685, sounds like some electrical diagnosis required on the actuator head.