Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove combustion block from boiler
Remove left-hand side case if required

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. A new gasket shall be used when the burner/combustion block is refitted.