Set Appliance State:

Mains Power Off
CH Demand
DHW Selector in Minimum (-) position
CH Selector in Maximum (+) position
ON/OFF Switch in ON position
Ensure boiler is not up to temperature
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler respond to a demand for CH?
Note:
A radiator symbol will also appear in the display if the CH demand is recognised.