I'm so excited to have happened upon this thread! And thank you, bumplbea for showing your photo!
Yesterday I took a picture of a plant I have never seen before. The photo is not in focus, and I wanted to get one with the flowers open, but I think it might be the Common Centaury (Centaurium erythraea) or Centaurium pulchellum or Centaurium calycosum, now called Zeltnera calycosa.
But you've all given me something to go on. It won't be labelled as 'Unknown flower' for long.