Ian, I should think the seed most likely was from a white form of incisa (and a good one) to throw a good clean white seedling like that so readily - a good white like that would already have to be already fairly dominant in the genetic makeup of the parent, I would think.
I agree about the reluctance of N. cyclamineus to increase by bulb division, which is why I added the proviso if it's a good grower, by which I really meant a health plant and a good increaser. That's the problem with a lot of named snowdrops; many are not strong growers or increasers. Still, even if the cyclamineus isn't a natural increaser, you could always summon up the courage to chip it. In the long run chipping can actually bulk up a good bulb much faster than division. Although I know it's gratifying if your bulbs clump up fast all on their own. Let's hope it makes a good fat bulb that'll at least chip easily. It really is distinctive.