Set Appliance State:

Mains Power Off
Heating Switch ON
System calling for heat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

At rest is the Servo Valve in the CH position and does it remain there when there is a call for heat?
(the cam should be in the fully returned position fully to the right looking at the front of the appliance)
Note:
The image shown is the rear of the valve looking at the front of the appliance the cam should be fully to the right.