I noticed this rather vivid yellow flower on our river side walk this morning.    There are 4 petals on the flower heads and the plant is only about 25/30 cms high and quite dense.   
I don't know what it is and cannot find it in my wildflower books nor on the Internet.   Perhaps someone can kindly identify it for me please.
It's called Crosswort and fairly common in Northern and Eastern England. (Thanks Miranda for identifying that for me)