Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect the wires as indicated by the red arrows and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler fire with no demand ?
Note:
The external controls may be wired into the terminal block different to that shown, therefore you may need to trace the wires for the external controls.