Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Disconnect the timer and fit a wire link between the two left connections on the 3 pin connector (as shown in the image)
If available use timer link supplied with boiler
Ensure links are still in place from previous test
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler now fire on a heating demand?