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
Mains Power On
Measurement:
Pre-heat switch ON.
Measure voltage across connection D25 marked PUMP (brown wire) on 12 pin connector towards top L/H corner of main PCB 23 and neutral (blue wire) on main connector block.
Measure voltage across connection D25 marked PUMP (brown wire) on 12 pin connector towards top L/H corner of main PCB 23 and neutral (blue wire) on main connector block.
Isolate as per TOPs P4
and refit as necessary