Hedgerow Crane's-bill (Geranium pyrenaicum) by John Pilkington at 10:54 pm 25/09/12

Confidence: It's likely to be this, but I can't be certain.

Notes: Pink-purple flower with 5 notched petals and darker "veins"; sharp sepals; crane's bill-like fruits with small thin petal-like protruberances; small upper leaves with very short or no stems and toothed edges in clusters; lower leaves larger and deeply lobed.