Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure boiler is cool
Drain primary system water from boiler
Remove casing cover for the non-return valve
Remove the non-return valve

Observation:

Inspect the non-return valve for restrictions and/or blockages.
Check the operation of non-return valve, gently push the center of the valve, its operation should be smooth and easy to push.