Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove combined fan, gas valve and combustion chamber assembly from the top of the primary heat exchanger
Remove the burner

Observation:

Thoroughly inspect the burner assembly for damage, restrictions and/or blockages.
Note.
1. If burner inspection is satisfactory do not reassemble proceed to next inspection.
2. New gaskets shall be used when the burner/combustion chamber assembly is refitted.