Right, sussed out the 12v socket issue for the nav.
If you have the usb socket fitted in the dash like mine does, that actually goes into sleep mode about 30 seconds after the ignition is switched off.
So I will order an adapter for now that converts cigarette plug to usb.
This will do for now, and when I get more time, I will pop out both the usb socket and the 12v socket that are in the dash and see if the plugs are the same, so I can just swap them over..