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Many thanks Paul. OK, so this is the Yellow swarming fly... notorious for huge gatherings in roof spaces. There are suddenly masses in my garden now it's warmer, so maybe my attic was full of them over the winter. Today some (presumably males) were sitting on leaves, running around and then fanning their wings when another fly passes close by or lands. Some flies (maybe females) fly very slowly close to bushes where males are sitting and occasionally fanning. I took some photos of this, and can see that the flies that fan also evert 5 yellow vesicles form the tip of the abdomen, almost certainly for releasing pheromones. I assume that these are males doing this and maybe the last photo posted below is a female with a fatter abdomen? Or is there a surer way of sexing Chloropidae. It might be a male before it has everted the vesicles and maybe females haven't emerged yet as I haven't seen any mating yet. I thought this might be a new observation but there is at least one study which suggests pheromones are involved in the winter aggregating behaviour as well as courtship. http://www.britannica.com/bps/additionalcontent/18/36877837/Male-flies-with-yellow-balls--New-observations-on-the-eversible-vesicles-on-the-postabdomen-of-male-Thaumatomyia-notata-Diptera-Chloropidae