I have now studied the underside of the "plug" and the wiring is as follows:
Orange wire to Pin 1 - Right motor
Orange wire to Pin 8 - Right rail
Black wire to Pin 4 - Left rail
Black wire to Pin 5 - Left motor
Black wire to Pin 2 - Rear Light??????????
Black wire to Pin 6 - Front Light??????????
If I understand FB's post correctly the wires currently attached to Pins 2 & 6 need to be re-attached to Pins 4 & 8 along with the wires already attached to these two pins. There could be a slight problem with identifying which black wire (left or right) from the front bogie, is then the correct black wire at the plug end?