Update – It seems to be a short form M. guttatus seeds (M. alsinoides is possible to show up in the mix). It has flowered for a long time in the summer in a part shaded position; very showy.
Showy when in groups and it should reseed if it likes the place.
Germination: probably best to sow it in place in late fall in a part shaded edge of the rockery, rocky area around a pond/waterfall, or other similar locations.
Update: seeds germinate in about 3 weeks at 4-8C (in a cold garage).