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 burner gasket or ceramic face of the burner.
2. Before refitting the burner inspect the
sealing gasket and replace if damaged.