A flower of small yellow balsam, Impatiens parviflora. I first noticed this plant growing alongside the paths under the trees in The Wilderness where it tended to have large healthy leaves but scrawny little yellow flowers:
I was finding it very difficult to identify this plant until I noticed the serrated edges of it leaves were very similar to those of Himalayan balsam (Impatiens glandulifera). However, while Himalyayan balsam can grow to over head-height, small yellow balsam grows only up to knee- or waist-height.
Photos taken in The Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2010-07-30 (first and last) and 2010-08-01 (second).