Phylum: Cyanobacteria   Family: Aphanizomenonaceae

Cylindrospermum stagnale É. Bornet & C. Flahault 1886

First described by Bornet & Flahault (1886: 250). John et al. (2011: 131) note “This is the most aquatic of the widespread species in the British Isles, sometimes on bottom sediments in ditches and ponds, but often mixed with submerged plants, either floating free or epiphytic.” It’s this entanglement that makes it difficult to photograph.
Cell dimensions: L: 4-7.5 µm; B: 3.8-4.5 µm. Heterocysts: 7-16 x 6-7 µm. Alkinetes: 31-40 x 10-16 µm.
Fairly common with marginal-vegetation of lochs, pools, and ditches.

References: 
Bornet, É. & Flahault, C. (1886). Revision des Nostocacées hétérocystées contenues dans les principaux herbiers de France (quatrième et dernier fragment).
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition). The Freshwater Algal Flora of the British Isles.