Wiggy,
Thanks for taking the time to reply.. It makes sense.
I think the key thing to remember these policy's will and do differ between airlines as all sorts of things come into play.
It's not just about fuel but a whole host of other services and costs that maybe affected by a change in CI.
I would suggest that many airlines don't even know their true cost index, this is highlighted in one of the Airbus getting to grips documents.