Phylum: Charophyta   Family: Desmidiaceae

Cosmarium regnellii Wille 1884

Originally described from Brazil, it has become something of a taxonomic mess. Kouwets (in manuscript) has made great strides in sorting it out. I'll make no further comment here until his work his published.

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.