Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
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)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance as necessary (hold down both arrow buttons until display changes).
Allow boiler self-check to complete and ignition sequence to begin.
Measure the voltage between in 1 (brown) and pin 2 (blue) at HV2 on the PCB.
Shortly after the fan starts mains voltage should be present.
Is mains voltage present during what would be the sparking stage of the ignition cycle?
Note:
Voltage will only be present for approximately 5 seconds for each ignition attempt.