Phylum: Rhodophyta   Family: Rhodomelaceae

Polysiphonia fibrillosa  (Dillwyn) Sprengel, 1827

Common on the lower shore to the intertidal, growing on hard surfaces and also other seaweeds

References: 
Maggs & Hommersand (1993) - Seaweeds of the British Isles, Volume 1 Rhodophyta, Part 3A Ceramiales