Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Measure the DC voltage between the left hand terminal of the 24v external controls thermostat and the earth point on the boiler chassis.
Expect approximately 24 Vdc.
Expect approximately 24 Vdc.
Isolate as per TOPs P4
and refit as necessary