Crowfoot, growing from within a small natural woodland pond.
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Looking again - having had the necessary information pointed out - I agree it looks a lot more like Ranunculus omiophyllus. I can see what you mean about the cut of the leaves. Thanks cicuta 58.
Lat/Lng: 51.796765702234, -3.6892175674438
OS grid ref: SN836122
Nant Llech (SSSI), the woods behind Wales Ape and Monkey Sanctuary, near Henrydd Falls.