Set Appliance State:

Mains Power Off
Boiler mains switch ON
Boiler CH thermostat to MAX
CH demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and the appliance to run in CH mode.
Does the system heat correctly?
Also monitor the system pressure as the CH warms up.