Set Appliance State:

Mains Power Off
Gas Off
Isolate Mains Power to appliance as per TOPs P4
Boiler ON/OFF Switch OFF
CH Temperature Selector OFF
DHW Temperature Selector OFF
Boiler discharged and drained
WARNING! Although boiler is discharged and drained some water will be left in the appliance care must be taken to prevent water damaging the electrical components or surrounding areas.
Remove CH & DHW pumps

Observation:

Inspect the Non-Return Valves and manifold for free operation and blockages.
Note: Inspect both Non Return Valve as boiler is drained.