Considering the location, a drift on a bank in the local woodlands, and after ruling out other possibilites (e.g. sweet woodruff) this is identified as Dog's Mercury.
The drift of Dog's Mercury
Spear shaped serrated leaves, flower spikes similar in form to nettles all fit. It is a poisonous plant which has little use. Hence the "dog" tag. It has been known since Pliny's time and supposedly was used in divining the sex of an unborn child. Hence the Mercury (messenger). Both parts of the name are up for dispute though. Dogs do make a bee line for it - and then start salivating copiously and then vomiting as can be vouched for by numerous dog owners online. So the association may be with the animal after all! At least the induced reaction ensures it is not fatal, and this hold for humans too.
