MEIOSUMYZA RORIDA FLY
NatureSpot tells us it has been found in various habitats including meadows, woodland edge and post industrial sites. May to October. Widespread and fairly frequent in Britain. Our example was found on the upper Leven in August.
It also notes that firm identification is difficult. Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.
Wiki adds : Females lay their eggs in rotting leaves, which the larvae feed on. Adults are active from May to October. They mainly feed on nectar and pollen of Heracleum sphondylium. The flower mentioned is common hogweed. While this one was not on this plant, it was growing adjecent to it.
WIKIPEDIA : https://en.wikipedia.org/wiki/Meiosimyza_rorida