Phylum: Charophyta Family: Desmidiaceae
Cosmarium venustum var. hypohexagonum West
This variety is quite distinct and would seem genuinely rare, with few British records. The zygospore had never been recorded until this 2020 find. The species has spawned many varieties over time and Kouwets (in manuscript) is working on a major revision.
References:
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium