Phylum: Charophyta   Family: Closteriaceae

Closterium dianae var. arcuatum  (J. Ralfs) L. Rabenhorst 1868

It differs from the nominate variety in having a greater curvature and also, the length is a little longer (but dimensions do overlap).
Cell dimensions: L: 277-332 µm; B: 24-26.5 µm; Ap: 3.4-4.3 µm; Arc: 142-152°; L/B: 12-13.
Fairly common in acidic peaty pools, including with Sphagnum.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.