Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4

Observation:

Undo the gas connection below gas valve, the 6 nuts around the edge of the burner door and carefully remove the burner assembly from the boiler.
Inspect burner assembly for signs of damage or blockages.
Before refitting the burner assembly check the seal is in good condition and replace if needed.