Puncture

[Phil TK]

So it's got 30mm at the tightest and about 50mm at the slackest. It's an "O" ring chain and always oiled so why so much variation? or it this normal?

Completely normal, in fact all chains are like this after the first few hundred miles. It won't cause any problems as long as you adjust the chain to the correct amount of slack at it's tightest point.
Good to hear that you use oil on the chain (as recomended by all chain manufacturers) rather than throwing money away on useless cans of 'lube'