Phylum: Chlorophyta   Family: Chlorellaceae

Mucidosphaerium pulchellum  (H.C.Wood) C.Bock, Proschold & Krienitz

Common in low nutrient waters such as the plankton of locks, bogs and pools

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