Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate flow and return valves
Discharge appliance primary water
Disconnect electrical connector from diverter actuator
Remove hydraulic cover by releasing the locking pin and withdrawing
Gently ease the non-return valve from its seating

Observation:

Inspect the non-return valve for contamination and damage.
Ensure there is light spring tension to confirm correct operation.
Note:
The non-return valve is fitted in the right hand side of the hydraulic section.