This little 2mm beastie showed up in my net in mid-June from a hedgerow sweep. It sems to key out as a Chloropid, and seems very reminiscent of the Thaumatomyia that show up around here later in the year, except that in this case the costa goes as far as vein M1+2. Mostly yellow but with sharp black markings, including the ocellar triangle and the base of the hind tibia. I want to say Chloropid but the experts may like to differ. Unfortunately it was too small for me to pin with ease and in carding it the specimen landed upside down!
The colouring is reminiscent of Gaurax fascipes (one of the yellow and black oscinellines), but I'm not very familiar with this species, so best to wait until the Ismays comment.