Okay, I still have one issue.
vertical layer lines. They're pretty obvious and consistent.
I'm guessing this is down to the z-steps/motor/screws not moving in completely consistent heights.
Given that the final height of the object is about as accurate as you can get, and the x & y dimensions are now also accurate.
Why am I getting these lines ? I'll pop pics up later.

I believe my gantry is level. It doesn't make any noise other than motor whirring when I move it up and down. Which i believe it would if it weren't level.
Also I can't see how that would make these lines anyway as they are consisten through the model, not just on one side or the other and the height is even and accurate on both sides.

This is my first I3 - and it's not an issue I've seen on any of my other machines or can remember cropping up on the forum. So currently kind of stumped.
Hopefully pictures will help someone go: 'Ah ha, I know what that is !'
:-)