Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Allow boiler to cool, running the tap to clear some heat will help speed this up
Reset the boiler when cool if necessary
Attach temperature clamps to the pipes from the heat exchanger - one on the flow pipe under where the thermistor and overheat are situated and one on the return pipe behind it
Boiler DHW thermostat on MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Fully open the hot water tap.
Allow the boiler to fire and the flow temperature of the boiler to rise to its maximum of approx. 65-75°C.
Check the differential temperature. It should be between 11°C and 20°C.
Note:
The boiler may lockout again in overheat. If you haven't managed to take a measurement you will have to repeat the test.