Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Rotary Switch to DHW position
DHW Temperature Control To Maximum
Hold in Boiler Case Switch
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Whilst pressing Ignition button, measure Mains Voltage across uppermost White wire on Air Pressure Switch and Neutral (Blue).