Ok, so tonight I tried again. The PC had been off all day and it was cool in the house. Tried to set it off on a render straight after booting up, and it got to 62% and just stopped again. Once again, it was at a different place to the other times its locked up. The time remaining countdown carries on, but the preview is stuck on a particular frame. The rest of the PC is useable. You can cancel the render at that point, and it says render cancelled but its then locked up.
Kept an eye on the CPU temps and they were running around 30c and peaked at around 37c. Case temps also seemed fine and stable.
As I say, when it freezes, CPU use drops down to 0% but memory use is still up at 90%+ until you kill the application in task manager, then the memory is freed up.
It’s mega frustrating.