Forgot to post that... and since then...
Relieved that the new one hadn't be nicked from my doorstep (which is literally on the road) for five hours (not even a pavement, as the road's not wide enough).
You can tell that this smaller one is lower spec, in that it took a couple of hours to go through all the commissioning and updates with the CPU on 100%, but now that's done, all seems to be working fine, and no sudden restarts. Even better, I found I had a spare 128Gb Micro SD card which is now being formatted so it can be extra outboard memory. TBH, not that much gets saved onboard now, apart from photos after editing, so I suspect the 128Gb of onboard memory won't run out soon, but it's nice to have auto external backups anyway.