Right... mystery solved. It was all down to me being a bit of a muppet. I was looking at the CPU sensor on the mb instead of the actual CPU temp This morning it’s idling at 25 with the mb CPU sensor at 17, which is pretty much room temp. 25 seems like a more realistic idle temp. The CPU temp is 45 when in the BIOS. If I boot windows and run hwinfo immediately I see the temp at 43.8, then it drops over time down to 24.8.