Thanks to an initial suggestion by Rex (thank you!!), I reseated the thermal pads on my CM5 after having constant issues with huge heat spikes up to 80 degrees. Having a small pad on each chip separated by a small gap sorted it.
He also mentioned using an M2 heatsink on the rear cover and bgav pointed me to this thread. My heat issues are fixed with these two suggestions.
I placed one M2 heatsink centrally, just above the middle screw on the rear cover (see pic) and ran cpuburn for 10 minutes. This terminal run app takes the CPU to 100% constantly, giving you the chance to monitor the temps.
In the 10 minutes, with one M2 heatsink on the rear, the temp of the CPU hit 59 once and stayed at 57-58 throughout.
As there are four in the pack, I added another heatsink just above the first and ran the test again after letting the uConsole cool down. This time the temp stayed at a max of 58 throughout the 10 minutes. Much better than the 80 degrees I was seeing before!
I haven’t tried underclocking and if the temps stay lower than 60 degrees, I’ll happily leave things as they are.
Thanks Rex and bgav 
Here’s a piccy of the rear cover with the heatsink box for reference. Available on Amazon for £8: M2 Heatsink
