I've narrowed this one down to two possibilities. Either Mecocnopsis cyclamenoides, or Cyclamen meconopsifolia. Maybe should crosspost into Bulbs.
Not quite sure what / when / how to intervene here. I have a dreadful track record of losing Meconopsis seedlings when I'm pricking out, hence leaving it till it was a good size.
The Cyclamen is a LOLA, but probably just a nice hederifolium that I decided was worth its own pot.
Also, I've quite a few Meconopsis, but I never get self-sown plants. Probably.