Thanks for all the suggestions guys, I've given up! I'm about to fit a marine socket on the rear bulkhead above the transmission tunnel. If I ever find the original I'll have 2 sockets!
Of course when my 1992 plus 4 was built people were not gadget mad. I fitted a 12v socket to the rear bulkhead directly from the battery but put in an inline fuse. From this socket I can run sat nav or what ever but also connect my battery conditioner.
