Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Remove diverter valve head from the appliance and place in a safe location
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to fire in CH mode.
Inspect the diverter valve spindle, it should be in the fully extended position.
Is the spindle in the correct position?