This doesn't sound right to me.
Is a 370Z worth £12K as a Cat D after two years? Well, possibly. However, if I were selling there's no way I'd take a £4K hit on the first offer unless I knew that actually I was getting the better deal. Even at £16K as a Cat D it's good value IF that was the only damage.
Now, the other scenario here is that the insurance co took a gamble on making a bit of cash. They wrote it off and paid out on the basis that they could repair and sell for more than that. I find that to be highly unlikely, but not impossible.
How much is a new bumper and sideskirt from Nissan, painted and fitted? About £4K? Hell, let's double that for the sake of a bit of fun and call it £8K to fix. They usually won't write a car off below 50% of the value, so they reckon that a ONE YEAR OLD 370Z WAS ONLY WORTH £16K?? Yeah, do the sums on that and something clearly isn't ringing true here.
Don't get me wrong, if we're all barking up the wrong tree then this is possibly the best Zed bargain I've ever witnessed, and well done sir. All I would say is to be very careful, and get a Zed specialist to look the car over as well as the AA guy (the AA will be quite generic, you really need to get someone who knows what they're looking for to have a nose). Even if that costs you another £200, if it checks out then it's still a bargain.