Set Appliance State:

Mains Power Off
Gas On; Pilot On
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Rotary Switch in DHW position
Keep gas valve control knob depressed
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage across one terminal of the overheat thermostat and the boiler chassis.
Repeat the test using the other terminal of the overheat thermostat and the boiler chassis.