Hey Graham,Had this same issue with my JLU Rubicon today. First time. Have around 3000 kms on it. Backed in the driveway and stayed between on and acc. Said vehicle not in park. Sat in it for a few mins while I read your post and then tried again and it shut off. Probably a total of 15-20 mins. With a few cycles of starting it and moving it.
Did you ever find out what was causing it?
It was a wire that goes to the shift lever to determine if the Jeep is in park. It took about 3 weeks time, but my dealer narrowed it down to something like a butt connector that was not crimped right. The only way to get to it to fix would have been to remove the whole dash, so they ran a new wire. It was very intermittent and hard to reproduce. Once they saw it, they were poking around til they found the issue and could reproduce/resolve by pushing on that wire.