Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Ensure that the Boiler Thermostat is set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the fan to run for approximately 10 seconds and measure the voltage across the black spaded wire and blue connections on the gas valve.