Yellow flower in park

Yellow flower in park

Observation - Yellow flower in park. Description: Park is full of these at present, in untended areas, for example behind fences and on river banks. Not a buttercup or dandelion, but what is it? Not a very good photo either - apologies.

Park is full of these at present, in untended areas, for example behind fences and on river banks. Not a buttercup or dandelion, but what is it? Not a very good photo either - apologies.