Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove fan/manifold assembly
Remove the burner/baffles assembly

Observation:

Using a torch and inspection mirror, inspect primary heat exchanger flueways for debris, damage and good general condition.
Notes:
This inspection requires the air/gas manifold and fan assembly to be removed therefore the gasket between the gas air manifold and the heat exchanger must be changed.
If space allows the heat exchanger can be inspected from above by removing the inspection panel on the top right.