Phylum: Charophyta   Family: Elakatotrichaceae

Elakatothrix genevensis prox.  (Reverdin) Hindák

Not well recorded and could be genuinely scarce. The image depicted has a question over its size, hence the epithet 'prox'.

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