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definitely Nemopoda nitidula. I found a pdf with drawings of genitalia and keys:

Pont & Meier The Sepsidae of Europe
https://www.researchgate.net/profile/Rudolf_Meier/publication/263429542_The_Sepsidae_Diptera_of_Europe/links/0046353ad408a4937d000000/The-Sepsidae-Diptera-of-Europe.pdf

page 127 shows the hypopygium which is easily matched with my specimen.

Thank You very much Sir Paul :)