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)
Boiler On
Make sure that any programmer/timers are on
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the AC voltage between D2 on Distribution board and Neutral connector.