This is the hybrid between the sharp-flowered and jointed rushes and can often be found where the two species grow together. It is a sterile hybrid.
Probably overlooked and under-recorded.
All names: Juncus acutiflorus x articulatus (J. x surrejanus) Druce ex Stace & Lambinon; Juncus x surrejanus Druce ex Stace & Lambinon; Juncus acutiflorus x articulatus