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Thank you all for your useful comments. I have made a copy of the Whitworth paper. I used this key HERE to ID the species but obviously I went wrong somewhere. I do have the specimen and will get to work this evening on its correct identification and look at other specimens to see if I do have a vomitoria.
Incidentally these 2 flies and dozens of others (not all collected) were common in a rotten-fish-bait trap in my garden this past May.