This appeared to rise up from the water and hang on a branch. It didn't appear to want to fly very much.
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Of the three British alderfly species, I'm pretty sure Sialis lutaria is the only one which breeds in still water, so if this beast is abundant next to one of the ponds on the site, that's the likely identification. You can identify males with the naked eye, as lutaria has a big triangular flap sticking out under the tip of the abdomen, which the other species do not.
Thanks Brian, this beast was certainly next to one of the ponds on the site. I'll make a note of looking out for these beasts again next year for a closer investigation.
Lat/Lng: 51.9, -0.7
OS grid ref: SP9329