Phylum: Charophyta   Family: Desmidiaceae

Actinotaenium didymocarpum  (P.Lundell) Coesel & Delfos

West & West (1904), as Penium didymocarpum, has it quite widely distributed but doesn't note the habitat. It would seem to prefer acidic moorland sites

References: 
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1
Kouwets, F.A.C. (1999) A check-list of desmids (Chlorophyta, Zygnemaphyceae) of France