Sorry - should have said. I've already replaced the 12V battery with a brand new one - and checked and charged it with a decent quality battery charger (CTEK 'Time to Go'). We've had the fault codes cleared twice and both times they've come back. It's then been to the local Nissan dealership...
I have a 2016 Leaf 30kWh, with less than 22,000 miles and a full 12 bars on the battery, but it has started reporting these fault codes:
P0AA6 high voltage battery system isolation, and P31E7 restart inhibition
Our local dealership has checked it out and has said it's not covered by warranty...