Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain Boiler Primary Circuit
Remove thermostat phial and phial pocket

Observation:

Inspect the filter within the top plastic manifold.
Is the filter clear and unrestricted?
Note:
Access can be improved by removing the PRV (as shown) and, if you have access, the side panel may also be removed.