Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove burner assembly

Observation:

Inspect main burner injector for restrictions.
Inspect around the injector assembly for any signs of soot or discoloration which may indicate the cork gasket behind has deteriorated.
Note:
If in doubt remove the injector / pilot assembly and visually check the cork gasket, however be aware if the gasket has not been recently replaced then this may damage it.