Thanks very much for the help. It's an Asus motherboard and they seem to use non standard names, but I seem to have managed to find the right setting, and HWMonitor is now reporting VCORE at 1.28v.
Since then, highest temperature listed under the CPU is 82, and I've had no crashes playing Borderlands 2 (previously tonight, it was crashing every 5 mins when trying to play).
Still weird that this has only just started to be an issue. (Well, it has previously crashed very occasionally, but only now started to do it very frequently.) And is 82C still rather high - just not high enough to crash it? So maybe still worth re-seating the cooler.