Tufted Vetch, or Cow Vetch. Photographed on the Thames bank again, at Reading. This is a member of the pea family, and is thus beneficial (often) to surrounding plants, by fixing nitrogen in its root nodules (remember your biology 'O' Level?!)
Cattle like Tufted Vetch as food, as strangely enough do pet budgies - which will readily eat the seeds and leaves.
Click photo to enlarge, note the leaves (very distinctive of peas and vetches) and also the SMOOTH stems. (Hairy Vetch looks identical to Tufted Vetch, but has hairy stems).