Barren Strawberry (Potentilla sterilis)
I hadn't identified this but a few days after photographing it I was in Waterstone's and I flicked through a book on flowers that I don't own and one of the first pages I came to was the Barren Strawberry (Potentilla sterilis) and I thought "ah!" so I came home and looked it up in my own book and lo, the description matches, especially the part about the "terminal tooth of end leaflet being shorter than adjacent ones". So a few days later I went back to check the leaves and it is indeed a match!
I probably wouldn't have seen it in the first place if I hadn't decided to follow a bee...
Wellfield Wood, Stevenage, 9 April 2010