Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the burner from boiler (if not already done)

Observation:

Inspect heat exchanger flue ways for general condition and cleanliness.
Note.
1. Ensure that when removed care is taken not to damage the surface of the burner.
2. When the burner is refitted or replaced the gasket shall be renewed.