Phylum: Chlorophyta   Family: Oocystaceae

Willea rectangularis  (A. Braun) D.M. John, M.J. Wynne & P.M. Tsarenko

More commonly known as Crucigeniella rectangularis it’s the commonest member of the genus. Found in groups of 16 or more cells (syncoenobia), they inhabit ponds, lochs and even slow-flowing rivers.

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