A tiny grey and black fly that looked like it might be laying eggs on fungus growing on rotting logs. Probably a female Polyporivora picta (Diptera: Platypezidae). Mainly yellowish legs, weak bands on the abdomen and a black scutellum (not shown well here). Identified by Menno Reemer in the comments below.
The following fly which I photographed in the same area just 4 minutes later is a female Polyporivora ornata. White scutellum, stronger bands on the abdomen and mainly black legs (not shown well here).
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2011-09-25.