Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Using a mirror and a torch look at the rear of the servo valve, there should be a 0.2mm gap between the top of the spindle and the bottom of the cam.
Is the 0.2mm gap present?
Note:
The image shows the rear of the servo valve and the spring has been removed for clarity, this test can be carried out with the valve still in place.