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Using the keys in Spencer's RES Handbook Vol 10 Part 5g (free download from http://www.royens...art05g.pdf), in Agromyzidae (Diptera) of Fennoscandia and Denmark and the info at http://www.ukflym..., http://www.leafmi... and www.bladmineerder... I think that this is Phytomyza petoei Hering, 1924. There are currently no records for this species on the NBN Gateway, presumably due to under-reporting. In the RES Handbook for the Identification of British Insects, Spencer gives its distribution as "Probably not uncommon in south".
I am hoping that this ID can be confirmed or corrected here.

Best wishes,
Martin