Phylum: Cyanobacteria   Family: Oscillatoriacea

Lyngbya majuscula Harvey ex Gomont

Forms mats 2-3 cms across either anchored to rock or free-floating. Fairly common the the upper tidal zone

References: 
Newton, L. (1931) - A Handbook of the British Seaweeds;
John, D.M., Whitton, B.A. & Brook, A.J. (2011) - The Freshwater Algal Flora of the British Isles