No Hot Water - Unvented System

[terryplumb]

The actuator is also referred to as the "head". It's the big silver box .the valve body is the brass T shaped bit below the actuator. The whole thing is the complete valve.
There is a small motor inside the actuator ,which is replaceable.
It's impossible to know what exactly is the cause of your intermittent fault by just reading what you have told us ,but My advice would be change the whole thing ,to ensure you don't waste time changing parts to possibly no avail.

 

[Shadoman]

OK. Thanks again for all the clarifications. It's been very useful for me.

 

[dilalio]

OK. Thanks again for all the clarifications. It's been very useful for me.

You could buy the whole valve and just try changing the head from it. If it doesn't work, then you can go further and change the body of the valve too.
This covers you for both events and the whole valve does not cost much more than just the head these days.