A female Anthomyiid fly, either Estalomyia festiva or Eustalomyia hilaris (Diptera: Anthomyiidae). Superficially similar to the much commoner Anthomyia procellaris, but the pattern on the thorax is actually quite distinctive:
This individual was continually on the move, walking over the upper and lower surfaces of logs, as if searching for something. It occasionally jumped from one log to another and only flew when I accidentally disturbed it.
Photos taken in the field below Chazey Wood, near Caversham, UK, on 2012-08-12.