Set Appliance State:

Mains Power Off
Selector switch to DHW position
CH temperature selector to minimum
DHW Preheat selector to ON position
Ensure there is a demand for preheat - this can be achieved by running the hot water tap with the gas to the appliance turned OFF
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With DHW preheat selector in the 'Preheat ON' position does the boiler fire without a hot tap running?