Thanks guys there's some interesting thought and debate going into this issue. Appreciate all your thought fodder, for sure. Though my original su*****ions if anything are slowly being confirmed amongst the noise - ie. my slackness with keeping the chain taught

Once again it was a relatively new KMC 'Z' 3/32" narrow chain; in the region of 600km (~ <400 miles) of life. It wasn't the master link that busted. Though it was only one link away from it (probably coincidence).

The photos illustrate that when it broke, the chain-ring threw the chain thereby also locking the drive-side crank at 9 o'clock as the slack was wound around the cog & hub locknuts. Also that the one remaining linkage plate at the break that wasn't lost shows that it was bent/torn outwards.

I wish I had my pics handy to post. I'm sorta trying to interpret what I can of the carnage myself - but experience from yourselves would of course be appreciated. I will post them from home tonight - in the a.m. US time.