Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Boiler drained and discharged

Observation:

Remove the pump venturi and inspect it for blockages and signs of damage.
Note:
Venturi is situated above the pump on the R/H side of the boiler in the return of the primary heat exchanger.
The venturi housing has two pipes connected to it that run from the pump flow valve.