Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Selector Switch to Hot Water and Heating
Attach two temperature clamps to the flow and return pipes from the heat exchanger, place one on the flow pipe under where the thermistor and overheat are situated. Taking care not to disturb the thermistors or the wires, and one on the return pipe behind it.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to fire and the flow temperature of the boiler to rise to its maximum of approx. 85°C.
You may find this will happen quicker if you close some of the TRVs/radiator valves off.
Check the differential temperature.
It should be between 11°C and 20°C.