Whilst out shooting with a friend and our XL-2's, we noticed what looked like a smudge on her lens whilst checking playback.

On closer inspection it would appear not be something on the glass but something missing from the glass, i.e the flourite coating ? It looks like a tiny chunk the size of a pea has come off.

Is this possible, has anyone else experienced it and what would be the likely repair bill to have the optic recoated ?

Generally only if either the coating is defective or certain liquids can destroy the coating

AFAIK the coating can not be replaced. The front element can. Generally front elements cost a few hundred plus labor. But i would check with canon

if it is still under warranty they may replace it under warranty if the coating was defective

if I'm correct, the coating isn't fluorite, but the actual element is. I don't think you can repair it - you'll need to do as Michael said and get them to change the actual element.