Lady's smock (Cardamine pratensis).
I remember these from Kemplah field in Guisborough, back when I was 5 or 6 years old. They used to grow in the long grass near a small pond that we used to play in (actually the pond was really just an overgrown puddle around a broken field drain). In the field, there were also buttercups, bees, and horses, and in the pond, leeches.
Alas, Kemplah field was long since built on, and a school has even come and gone on the site of the pond. This makes me feel old.
Anyhow, this is what the leaves of lady's smock look like:
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-04-28.