Any other suggestions or thoughts would be most welcome as this is driving both myself and the original installer to distraction.
Many thanks
I'm not a qualified plumber but you are welcome to my considered thoughts.
I would investigate the possibility that air is trapped within the DHW circuit in the boiler itself. I don't know what mechanism is supposed to vent air in this location, but I would look into that, and also the HE which may be flawed and have trapped air.
With air trapped as described, a closed valve on the HW outlet would make no difference, and a check valve on the cold water inlet would make little difference as the pressure blip from closing a cold tap would still pass the valve and briefly spin the aquasensor, (which sounds like it is working correctly).