Growing in horse grazed turf beside the tidal River Nene.Many other saltmarsh species present.
No interactions present.
I can see a resemblance to Bupleurum tenuissimum, but that has clusters of small yellow flowers within a whorl of prominent bracts.
On the otherhand, I'm not finding a native monocot with the right appearance, so it could be that I'm badly misinterpreting the image.
Lat/Lng: 52.696361, 0.156555
OS grid ref: TF458131