About 2mm in length, attached to the side.
No interactions present.
Thanks for pointing out that I'd selected a genus, whoops. I thought I was selecting a family name. This subject area is very new to me and very, very confusing ;)
Lat/Lng: 51.4413, -2.5946
OS grid ref: ST587715
Fast flowing, rocky, shallow river (the Malago) about 1 foot deep.