Set Appliance State:

Mains Power Off
Gas On; Pilot On
CH Demand
Rotary Switch to CH/DHW position
CH Thermostat To Maximum
Hold in Boiler Case Switch
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Run boiler up to temperature whilst monitoring system pressure on the pressure gauge.
Does the System pressure remain below 1.5 bar?