Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
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 wires at the molex plug running between the PCB and the gas valve.