Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH switch OFF
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Remove the grey plastic top cover from gas valve solenoid
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Remove the grey plastic top cover from gas valve solenoid
Gas On
Mains Power On
Measurement:
Pre-heat switch ON.
Allow fan to run and pilot to ignite.
Measure voltage across the terminals of the main gas valve solenoid (black wires).
Allow fan to run and pilot to ignite.
Measure voltage across the terminals of the main gas valve solenoid (black wires).
Isolate as per TOPs P4
and refit as necessary