Purple non-tulip (Triteleia laxa)

Those of you who know me well will be well aware of my lack of botanical knowledge - but this is even more of a mystery. 

When we had the extension porch built last year, the top end of the flower bed by our steps up from the drive was totally destroyed - literally down to concrete. Since, we have re-composted the flower bed, but have not planted anything yet up by the house. Yet these rather pretty little flowers have sprung up over the area - we've about half a dozen of them, and have no idea:

- What they are
- How they might have got there.

Any ideas, botanical blippers? - thank you!