Is it even necessary?
My piston crown has some light carbon, nothing chunky at all. A little bit on the sides too.

What is best to remove it, of should I even bother?

Is it still in situ? If yes, leave it be, if not, a stiff green scotch bright. Some also swear by oven cleaner, which makes a lot of sense.
Mike

Thanks Mike, it is. But whilst the barrels off I'll do it anyway.

Oven cleaner you say? Not sure if she even does the oven with that!

Yes, if it's still in the engine then rule out the oven cleaner option.
A stiff greeny and some WD 40 will do the job, but try to keep away from the very edge.
Mike