I did not do nuffink, honest!!
Erm, well, came to the conclusion that, looking at the tab washer after retrieving it from the sump, that I may not have pulled it up around the bolt head.
i am thinking that it worked loose, then once slightly loose, the tension of the chain snapped the head off.
SOOOOOOOOOO, called Alan Bird, and they had ALL, bar one, of the valves i need in stock, but when it happened originally I still had some straight ones!!
So, the bits will be here tomorrow!!! :D