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I found following criterion

"As can be seen from the accompanying photographs, Peplomyza litura is quite easily identified by the rather smoky-brown wings that are almost black in their rear half. The thorax is also streaked and the eyes are a rather vivid red. All the legs are entirely pale-coloured – there is one other British species of Peplomyza, P. discoidea, which looks very similar to P. litura except that all the legs are partly dark-coloured."
https://stories.rbge.org.uk/archives/23236

This is in line with the photos from P. discoidea presented at https://www.gbif.org/occurrence/gallery?taxon_key=1569881 and the photos for Peplomyza litura https://www.gbif.org/occurrence/gallery?taxon_key=1569883

Furtheremore the dark colouration in Peplomyza litura is less present in the frontal lower part of the wings when compared to P. discoidea.

=> Peplomyza litura.

Please feel free to disagree with me! I am just learning....

Thomas