
I had a look at the gears and the big one that drives the gear on the crank has some fresh marks on the outward surface (i.e. the surface facing you if you're looking at the gears in situ)... I'm not sure how this could happen

The battery may have been a little flat (I've started other bikes with far flatter batteries) when this happened - could the resistance from the engine be overpowering the starter motor and forcing it 'backwards' or popping a gear 'out' and skipping a tooth (presumably this how these gears lose teeth? Mine have all there teeth tho.)?
Not sure if this was every time I heard the sound, but I was heavily choking the air intake (to draw fuel through the carbs) - could that have something to do with it?
Cheers