Bolboschoenus maritimus Sea Club-rush CC DD N
This plant is very common round salt marshes, muddy seaside places and estuaries. The club shaped flowers on a sharply triangular stem, usually growing in large patches, are the things to look for. Until fairly recently it was known as Scirpus maritimus.
Except for the north coast of Scotland this plant is found on most of the coast of the British Isles.