Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure secondary heat exchanger is cool and there is a
demand for Preheat
Turn OFF the flow and return isolation valves underneath the boiler
Turn hot water tap ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the fan to run and the burner to ignite.
Does the hot water temperature improve? Or does the boiler temperature rise rapidly causing the boiler to shut down too quickly? (this may result in the hot water surging hot then cold)