Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Discharge the primary water from the boiler

Observation:

Disconnect the entire pipe as the rubber hoses may be brittle and break if directly removed.
Pour water through one end of the flow pipe and check for signs of restriction by accumulation of sludge in the hoses.
Repeat test on return pipe.
Note:
If the hoses have been upgraded to the modified all copper socket type, these should still be checked for restrictions.