Quaking Grass or Maidenhair
The name 'quaking' comes from the extremely mobile seed heads, which move in the very slightest of breezes. Once quite common, but I have never seen it on the washes. It will in fact grow quite happily in dry conditions - I have some in my garden.
Folklore. An old belief stated that this only grew in places where a young, usually love-lorn, woman had drowned herself, so this was yet another plant believed to be unlucky in the house. W H Barrett (Tales from the Fens) states that in his boyhood the old people maintained that it flourished particularly well wherever a corpse had lain, having been dragged from a river.