No. 1 Son came to me yesterday after we had had the smallest of blips on the main power telling me he had the three rings of death on his Xbox. I had heard about this problem and new it could mean a variety of things – the main one being the GPU heatsink. But this didn’t line up with it happening immediately after a power blip.
On investigation we noticed that the power brick was showing a red light (usually shows green when running, and orange when off). So we unplugged it from the mains, left it for five minutes and plugged it back in. Fixed!