The attached photos show a plant growing in profusion in the Weald of SE England, I don't believe I've seen it elsewhere (specifically, as one moves from the clay Weald up onto the chalk Downs) and suspect some variety of Tansy.
They appear to favour damp areas on the edge of pasture or in woodland provided that there's adequate light. Unlike ragwort, to which they are superficially similar, the seeds don't have "tufts"; also they are only a couple of feet high even when in flower. Flowering is roughly July through August, the photos shows the bud, flower and seedhead stages. The leaves are somewhat aromatic, slightly like wormwood.
Any thoughts would be appreciated.
Updated: I was wrong about the seeds. The seedhead matures into a brown pompom. Each seed is around 1mm long, ridged, with numerous hairs fanned out sideways from the top of the seed (i.e. no dandelion-style stalk).
