Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Selector Switch to HW position
CH temperature selector to Min
DHW temperature selector to Max
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across connection 1 (orange wire) on connector M1 on the ignition PCB and a suitable neutral.