Smooth Sowthistle, Sonchus oleraceus

. . . and if you look really closely, you might just be able to make out a couple of the common 'Marmalade' hoverflies - Episyrphus balteatus (male)

An hour after I took this, the flower had closed right up. Was it because its job was done? Was it because the air humidity increased (many flowers shut up if it about to rain)? Or was it because flowers of this species only last half I day (like goatsbeard)?
I shall have to monitor this plant over the next few days to find out as I can't find a definitive answer via Google.