Growing with moss on a dead trunk. Gelatinous and green!
No interactions present.
Have a look at this webpage: http://www.dgsgardening.btinternet.co.uk/bluegreenalgae.htm which I got to via the Encylopedia of Life for Nostoc. The last photo at the bottom of the page looks very like what we have both found.
Fantastic! Thanks very much!
Lat/Lng: 51.041166666667, -0.61083333333333
OS grid ref: SU974277