Okay so quick update, I was waiting on one of these things:
And it has arrived from China. Basically you shove it in your cigarette lighter socket and it cycles between showing your current battery voltage (this would be your car battery on an ICE car or the auxiliary battery on the Leaf) and the inside temperature which it cunningly measures with the little probe sticking out of it so it gets a more accurate reading. It also contains a 2A USB phone charger which actually charges at 2A. I know this because normally my phone, a Samsung Note 2, when on the charger actually discharges with googlemaps on. With this gadget it charges slowly.
The temperature and voltage measurement seems quite reliable (yet to be verified with a proper voltmeter) and its fairly well made for a £5 gadget. So basically I am impressed by the range of useful things this gadget does and the very low cost of it.
My lithium auxiliary battery does not exceed 14.4v so I am now not really concerned that it will come to any harm being used in the Leaf.