Stumped

With A, L and K away, I'm on twice-daily dog-walking duty, but this morning it was a pleasure since the stuffy, humid and cloudy weather had (temporarily) been replaced by a fresh, clear and sunny morning. For the past few weeks I've been thinking that this year's been good for thistles, and today I saw what appeared to be a whole field of them. However, on closer inspection this was the flower on an un-thistle-like plant.

Any ideas of what it is?
As TrikinDave says, "Phacelia, a green manure plant"