Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the Burner from the boiler

Observation:

Inspect heat exchanger flue ways for general condition and cleanliness.
Note.
1. Ensure that when removed care is taken not to damage the
ceramic face of the burner.
2. Before refitting the burner inspect the sealing gasket and
replace if damaged.