I've been asked to comment on this fly, which was found at Nursling near Southampton, Hampshire, on 5th July. I'm not competent in these Families, and would appreciate some input. Using Oosterbrook, it seemed to be coming out at Mycetobiidae with the basal cell arrangement, but I dismissed this on other grounds. I then had trouble working out the relations of CuA1 and the stem of M, and the relationship between A1 and the wing margin is out-of-view. At that point I must confess that I thought it looked a bit like Keroplatidae I'd seen in the past.......not a great way to identify flies!
This has already been identified as Macrorrhyncha flava (by Vladimir Blagoderov) on the UK Diptera Facebook page. Sometimes it easier to follow the key backwards!
Thank you Tony. What a strange turn of events! And thank you for your tip on using the key to Families.