Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Gain Access to the external controls wiring
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

This boiler is fed via a switched live directly from the external controls.
Therefore it will be necessary to eliminate the controls by testing between them and the fused spur.