Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH switch OFF
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Mains Power On
Measurement:
Pre-heat switch ON.
Measure voltage at connection D18 (violet wire) on 12 pin connector toward top L/H corner and the neutral (blue wire) on the 11 connector terminal block towards R/H corner on main PCB 23.
Measure voltage at connection D18 (violet wire) on 12 pin connector toward top L/H corner and the neutral (blue wire) on the 11 connector terminal block towards R/H corner on main PCB 23.
Isolate as per TOPs P4
and refit as necessary