My Norton flattening stone just arrived in the mail, I was thinking of ordering one anyway but Bruno's post gave me the required poke that I needed.

I haven't had chance to use it yet (job for the afternoon maybe), but on the instructions it says that when it eventually needs flattening itsself I'm supposed to use 220 grit waterproof sandpaper or a 'diamond sharpening stone'.

I'll report back later when I've tried it out.