Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to DC 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
Attach negative probe of multimeter to 0v pin (D32 upper of the two pins protruding from the right of centre towards the bottom of PCB 23)
CH switch OFF
Attach negative probe of multimeter to 0v pin (D32 upper of the two pins protruding from the right of centre towards the bottom of PCB 23)
Mains Power On
Measurement:
Pre-heat switch ON.
Measure voltage at D2 connection marked pre-heat (red wire on the 10 pin connector which is central to PCB 23 above the ignition PCB).
Measure voltage at D2 connection marked pre-heat (red wire on the 10 pin connector which is central to PCB 23 above the ignition PCB).
Isolate as per TOPs P4
and refit as necessary