Set Appliance State:
Gas Off
Set Multimeter to AC Volts 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in CH & HW position
Gain access to PCB as necessary
Selector Switch in CH & HW position
Gain access to PCB as necessary
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With a Central Heating demand, measure Mains Voltage across the Heating Control Thermostat (White/Black) and N (Blue) on the Appliance Terminal Block.
Isolate as per TOPs P4
and refit as necessary