Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Mains water OFF
WARNING!
Drain boiler before removing the DHW inlet filter
Locate the DHW inlet filter - this is situated in the cold water feed pipe connection fitting and accessed from the bottom of the appliance.

Observation:

Inspect DHW filter for blockages or damage (located at the base of the valve).
Replace filter after this test, re-establish mains water and re-pressurise system.