I live in the Uk and grow many forms of Galanthus, both species and named forms. Once planted I find they increase fairly quickly in a few years from producing one flower to ten. Nivalis and Woronowii seed around freely but to get a larger spread I would also dig up your clumps preferably while dormant and replant singly. It may look a bit sparse at first but you will eventually get the desired effect. But natural colonies take many years or even decades to establish huge areas of bloom. The photo is of a natural colony in a graveyard. Of course you could venture into twin scaling or chipping but that's a whole different ball game. Good luck though, it will be worth it.
