Insurance companies will normally not cover things such as vandalism, natural disasters, etc... If you're lucky, they'll meet you half way - Therefore Toffe, although your case is quite annoying, its not out of the ordinary in any way. I had that same issue with my M3, a stone flew at me on the motorway from God knows where, and left a little crack - little by little the crack started to widen so I had to get it replaced.
My view is, that if its not a serious problem, then nothing needs to get done... the front bumper of my M3 has a scratch on the side that happened when I sadly ran over a stray cat. Since the dealer wanted a fortune for a touchup job and/or bumper replacement (Labour is such a pain! They wanted close to a thousand pounds for labour!), I told them to forget about it since I realised it wasnt THAT serious and I wasnt prepared to dish out that kind of cash for a simple 'Cosmetic' procedure.
Mal - Where you from in Kent? My girlfriend's from Maidstone. Such a gorgeous place!