Small-flowered buttercup is grass under trees.
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In R. auricomus the flowers are frequently missing petals giving a rather assymetric look. Also compare the leaves on your plant with pictures of those on Geum and R. auricomus - you will see that they look like the former.
Lat/Lng: 51.072, -0.69
OS grid ref: SU918311
In rough grassland in a clearing of mixed broadleaved woodland - oak, chestnut, birch, some conifers, altitude 238m.