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Distichlis (sic) spicata is not known from the UK, and Spartina patens (fide Stace) is known from one site in Sussex.
There is Spartina (possibly S anglica) on the saltmarsh at Porlock Weir -it has been spreading over the past 10 years since the shingle ridge was breached by the sea and the area is now subject to "managed retreat"
Lat/Lng: 51.2, -3.6
OS grid ref: SS8848