Phylum: Chlorophyta   Family: Cladophoraceae

Cladophora glomerata  (Linnaeus) Kützing

This is one of the commonest algae. Found attached to a rocks and other firm substrates. In favourable habitat, it can form extensive mats

References: 
John, Whitton & Brook (2011) - The Freshwater Algal Flora of the British Isles