Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC 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:

Measure the voltage at pin 7 of X7 on the PCB to a suitable earth point.
Voltage should be approximately 1.7V DC.