Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between the black wire marked "GV1" and either of the blue wires marked "N" at the rightmost plug on the PCB.
Mains voltage should be present.
Note:
Two types of gas valve lead may be fitted, if you only
have one blue wire at the plug then use that one.