While doing some thermal debugging this weekend I stumbled upon the rather useful temperature monitoring utility "Psensor". I configured it to update stats every second and according to perf it was only using 0.02 CPU's worth of compute, so it seems relatively lightweight and shouldn't contribute to warming the machine up!
I like the min/max values being clearly shown and also the ability to change graph colours and toggle data on or off. Quick, easy and effective. Not sure why I haven't found this tool earlier, but I wish I had!