Used a battery to find out what each wire was. The centre contact was the common with the contact marked 'A' being the headlight and the other being the tail light. I've wired up the lighting as per the diagram for the plug (ESU, 21 pin Adapter Board) with A head and B tail wired to pin 8, A tail and B head to pin 7 and the common to pin 15.
I don't get any lights though!
I used a multimeter to test my wiring and I get a connection between the contacts and the corresponding connections on the plug so I'm thinking I may have missed something.