Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Flow and return isolation valves OFF
Boiler drained and discharged

Observation:

Remove the large brass sealing nut from the cyclone (take precautions as a small quantity of water will still be present)
Visually inspect the inside of the cyclone to check that is not blocked or restricted.
Is the cyclone clear and unrestricted?
Note:
1. In most cases this can be done in situ but if access is poor to the underside of the boiler then the cyclone may need to be removed to check it.
2. If the cyclone is clear and unrestricted then do not refill the appliance at this stage.