Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Off Flow and Return Valves
Drain Primary Water from Boiler
Remove Combustion Cover Case
Remove Condensing Heat Exchanger

Observation:

Holding condensing heat exchanger over a bucket use a jug to pour water in the flow connection at top.
After approx a cupful hold heat exchanger upright in its normal position.
The water should run out the return connection quite quickly as the pipe inside is in a downward spiral.
If the water doesn't run out satisfactory then this indicates a blockage or restriction