Yes, they came back to me fairy quickly in fact. Here's what they had to say.
Dear Andrew,
Thank you for your email.
I am really sorry to learn of the problem you are experiencing.
From your description it seems that the camera may have indeed developed a problem with power management, or a short circuit. This would explain why the battery seem to drain with the camera turned off.
Can you please test if the problem can be replicated after:
1. The camera firmware has been updated to version 1.13 (please note the update has to be done twice)
2. Memory cards, the lens and all other accessories have been removed.
Looking forward to your reply,
Kind Regards,
Nikon Customer Support
So, I've done as they asked, charged a battery to 100% removed everything (cards, grip, lens), and left the camera for 24 hours. Normally I'd get around 15-20% battery discharge per day with the camera turned off. I checked it before after just over 24 hours and the battery is still at 100%, which is a first. So I've now added the lens back and will see how that goes. If everything's OK then I'll add the SD card then the XQD card.
My gut is that it's the Lexar XQD card (a 64gb 2933x 440mb/s) unit which although is on the list of compatible cards by Nikon for the camera, I have noticed that when inserted, things like camera switch on and image review are not instant like they are with just the SD card, but introduces about a 1-1.5 second delay ?
We'll see.