Set Appliance State:

Mains Power Off
CH Demand
Ensure the appliance is cool (below 40°C)
Boiler thermostat set to MAX
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if it is in lockout.
Does burner ignite and establish smoothly?
Note:
The flame can be seen via the viewing mirror at the top of the appliance, some versions of this boiler do not have this facility, in this case listen for burner ignition.