Phylum: Chlorophyta   Family: Scenedesmaceae

Coelastrum pulchrum  Schmidle

One of a bewilderingly large number of coenobia algae, there are quite a number in this genus alone. Possibly common but rarely recorded. Identification confirmed by David John and Chris Carter

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