Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove Burner Assembly
Remove Sump cover Assembly

Observation:

Inspect the flue ways and the heat exchanger sump ensure it is clean and clear from restrictions and debris.
Note:
Upon reassembly check the integrity of any disturbed seals and replace as necessary.
While the burner assembly is removed it is worthwhile checking the condition of the Ionisation probes as per the safety bulletin.