Coastal variant of Water Whorl-grass, Catabrosa aquatica.
Open, feathery flower-heads. Only one or two flowers to each spikelet. Hairless leaves and stems, often purple-tinged.
Prostrate, rooting growth, to 50cm.
Western coasts on damp sand above high-water level. Usually found where a small stream meets the shoreline. Rare, but probably often over-looked. Identified by Alan Silverside