BIOS updated to version 2001, with voltage control left at auto - which it re-defaulted to.
Immediately I see that VCORE is sitting at around 0.8V and temperatures are much lower. Max CPU temp is now 40C! (Still to stress it, but that's at least a 20C drop.)
Thanks again to everyone in this thread. Without the link from temp to voltage, and from there to BIOS versions, I wouldn't have got to this point!