Thanks for everyone's input.
So to confirm Whole 3 way Valve was changed with Actuator. Immersion is definitely not running (was checked by engineer & if the tank stat is right down, we get no hot water).
I've been doing a bit more observation today -
When the Water is 'On' at the timer...
I have a puzzling problem with my heating system.
System comprises of Ideal icos HE18 boiler, Grundfos ALPHA2 Pump, Drayton MA1 Actuator, Drayton HT53 Tank Thermostat.
To cut a long story as short as possible:
I have a system that is generating Hot water that is far hotter than it should be...