Kernel_task high CPU even when booting from external

My 2019 27inch imac has suddenly developed an issue where Kernel_task is takig up a huge amount of CPU usage, rendering the iMac pretty unusuable.

I have reset nvram, pram, electricity supplier, colour of power leads etc, no difference. I also have booted from clean install external SSD drive and PCIe drive, all clearly so not to do with the same poor performance.

I have also booted without wifi or any network connection to ensure i's not a problem with icloud files.

I think this has shown itself after the power lead accidently was detached, so suspect some corruption in the basic system.

Any ideas folks!