Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove combustion block from boiler

Observation:

Thoroughly inspect the burner assembly checking for any damage, restrictions and blockages.
Is the burner satisfactory?
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.