Set Appliance State:

Gas Off
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Isolate Mains Power to appliance as per TOPs P4
Rotary Switch to DHW position
Hold in Boiler Case Switch

Measurement:

With Pilot alight (hold in Gas Control Valve) measure greater than 8mV between Black Thermocouple wire connected to Overheat Thermostat and Boiler Casing.