Well after much investigation, I can't explain this one.
That is annoying in itself, I always like to work things out and find answers.
Never had the problem before - just very odd that it was a BIOS message. After about half a dozen hard boots the machine came up and has been behaving fine since.
It certainly isn't a heatsink issue. I re-checked that with your suggestions Richard - thanks for that.
The Intel Motherboard Monitor software which comes with the board has not shown any thermal events.
A bit of a mystery.