Zumo 595 it is then. Sat Nav, bar end mount and the BMW connector arrived yesterday. Checking with a multimeter it seems of the three yellow wires coming out of the connector 1 is -Ve, 3 is +12v and it's switched by the ignition (eventually).
Only issue to be resolved is what to do with the yards of cable attached to the Garmin mount. Apart from the extra length it has audio in and out, Mic in and USB sockets that I don't need. I'm inclined to cut that back so I only have the 18" of cable needed to get to the connector in front of the battery; the mount is 'only' £40 to replace if I ever need one of the other connectors.