Saxifraga stribryni, a tiny saxifrage with lime encrusted leaves found on limestone mountains in Greece and Bulgaria. This is a two year old seedling in full flower in a trough. It's just a couple of inches high.
A few minutes in the garden before we set off for Southampton via London tonight yielded four butterfly species. A Brimstone settled next to me as I took this shot but was gone before I could focus on it!