Set Appliance State:

Mains Power Off
Ensure there is a demand to heat the thermal store - this can be achieved by running the DHW with power turned off to the appliance
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is the diverter valve head in or moving to the DHW position?
Note: The white plastic manual lever is fully towards the R/H side when the valve is in the DHW position.