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Lunaria rediviva
Tall "wooden" plant with leaves that look like flattened pea-pods. This is how a plant actually looks like in winter, but in summer it looks completely different (look for the picture in links on the right side).
Forests, near water-damp woods, on lime. Throughout Europe.
/from Wikipedia/: The common name "Honesty" arose in the sixteenth century and may be due to the translucent seed-pods which are like flattened pea-pods and borne on the plant through winter.
I don't know how I missed your suggested ID, but thank you! Actually in the summer there are really loads of these flowers and thank you for helping me find it! :)