Photo taken 23/06/08 near Mapledurham, Oxon., on Walk 20 of the Chiltern Chain Walk
I have seen this charming yellow flower a few times since, including a large swathe of it on a green on the edge of the Ashridge estate. It is common on waste ground and waysides, flowering from June to October.
Photo taken 13/07/06 on Icknield Way in Hertfordshire (Day 10 of my Berkshire Essex Walk).