Set Appliance State:

Mains Power Off
CH Demand
Control Thermostat in high position
High/Low Switch in High position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the Boiler to purge itself and the Burner ignite.
With the High/Low Switch in the High position does the High Flame Neon (Green) illuminate?