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leaves of Wood Sorrel Oxalis acetosella.
It's easy to be sure - taste one. They have a delightful sharp flavour, like a grape skin.
I have used them, sparingly, in salads. Do it with caution though: they are actually poisonous in larger quantities. The active ingredient is oxalic acid.
Lat/Lng: 50.1965, -5.1471
OS grid ref: SW754376