Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain boiler primary circuit
Unclip pump proving switch

Observation:

Remove the 4 screws from the pump proving spindle housing and inspect the diaphragm, spring and spindle for signs of wear or damage.
Inspect the main inlet inside the diverter valve and the weep tube for blockages or restrictions, a small Allen key will fit into the diverter valve side of the weep tube.