Set Appliance State:

Mains Power Off
CH Demand
Attach temperature probes to the flow and return pipes near to the boiler
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and boiler to fire. Let the system reach a reasonable operating temperature and then turn off the demand.
Measure the temperature of the flow and return pipes. Is the temperature differential 20°C or less?