Water-hemlock (Oenanthe crocata)

26 June 2020*

The water dropworts, Oenanthe are a genus of plants in the family Apiaceae. Most of the species grow in damp ground, in marshes or in water. Several of the species are extremely poisonous, the active poison being oenanthotoxin. The most notable of these is O. crocata, which lives in damp, marshy ground, ...

There is some debate over this flower, I was thinking Hemlock Water Dropwort, (Oenanthe crocata).
It is from the  Water dropworts family, need to take a better photo especially of the leaves.
Most Water dropworts are very  poisonous so need to show it a little respect!

*UPDATE*
Water-hemlock (Oenanthe crocata)
the leaves are definitely O. crocata, I think that other inflorescences have been cut/grazed and so it is producing atypically small and delicate inflorescences, I cannot see anything to suggest O. fistulosa (Twitter)