Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
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)
CH switch OFF
CH switch OFF
Gas On
Mains Power On
Measurement:
Pre-heat switch ON.
Allow fan to run.
Measure voltage across connections D12 marked N (red wire) and D14 marked GV1 (red wire) on the 7 pin connector, central of the three connectors towards the top of main PCB 23.
Allow fan to run.
Measure voltage across connections D12 marked N (red wire) and D14 marked GV1 (red wire) on the 7 pin connector, central of the three connectors towards the top of main PCB 23.
Isolate as per TOPs P4
and refit as necessary