A conopid fly, possibly Conops quadrifasciata, but don't quote me on that. This family of flies are parasitoids of bumblebees, jumping onto them when they are feeding and injecting an egg into the abdomen. The grub develops inside the bee, eating away the contents, eventually pupating inside to overwinter in the bee husk. Bees that have been infected may behave differently from normal bees. You can read more at http://www.bumblebee.org/PREDATORS.htm. This one was at Rowney Warren on ragwort. I saw several there including one of another species too.