Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain boiler primary circuit
Remove the two inline connectors to the pump

Observation:

Remove the four bolts holding pump in position, taking care that any residual water does not damage appliance.
Check the pump and housing are free from sludge or debris.
Check the impellor is in good condition and is free to turn.
Note:
The design of this pump means limpet cannot be used to check operation.