Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Thoroughly inspect the condition of the seal between the burner and heat exchanger, also inspect the condition of the primary heat exchanger casing and flue collector for any distortion.
Pay particular attention to the rear of the appliance.
Are the seals, gaskets and heat exchanger in good condition?
Note:
To confirm the condition of the gaskets and seals the use of a mirror and performance tester will be required.