Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach 1 temperature clamp on the return pipe inside the boiler just above the pump nut as indicated by the red arrow
Attach the second temperature clamp on the flow pipe to the radiators (furthest right hand pipe) after the isolation valve
Boiler CH thermostat set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite and the boiler to run in central heating mode for at least 2 minutes.
Monitor the temperature of the two pipes.
Is the flow pipe to the radiators hotter than the return pipe to the heat exchanger inside the boiler?
Note:
Ensure a good contact is made with the temperature clamps.