Phylum: Cyanobacteria   Family: Stigonemataceae

Stigonema minutum Hassall ex Bornet & Flahault

Common but easily overlooked given its size. Found on damp rocks and in association with lichens

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