Set Appliance State:

Mains Power Off
On/Off Switch OFF
CH Demand
Ensure DHW is satisfied i.e. storage cylinder up to temperature
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Appliance On/Off switch On.
Reset lockout as necessary.
All heating controls calling.
Does the appliance ignite and run with heating demand?