Yognuo 468 & 565 incompatible with Canon 550D
Thanks for pointing me in the right direction.
It turned out to be a near invisible pin, hidden under the right hand thin strip of spring on the hotshoe. Not at all obvious. When the flashgun is slotted into the shoe the spring strip is pushed down and the tiny pin hidden underneath is pushed into the camera body telling it the flash is present. I think a bit of dirt had got in and the spring wasn't fully returning when I removed the flashgun.
What a relief.