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Interesting.
Thanks Igor, I can try and collect in future.

The Danish institute collection of Icelandic Diptera includes Syntormon pallipes.
In total for this family it has :

14 x Campsicnemus armatus
340 x Dolichopus plumipes
1 x Hydrophorus viridis
5 x Syntormon pallipes

https://samlinger.snm.ku.dk/en/dry-and-wet-collections/zoology/entomology/diptera-collection/diptera_iceland_20020904_pdf.pdf

Mention of Hydrophorus viridis is also explicit in literature as being found on Skeiðarársandur, a glacial outwash plane :
https://utgafa.ni.is/skyrslur/2005/NI-05005.pdf
"On wet sandbanks above there is the stilt fly Hydrophorus viridis, which is rare in the country." (google translate)

However, Medetera diadema is marked on the list from the institute, in a way I think they mark for species not necessarily established here but just found once or twice.

The only other one I have seen here is what I believed to be Sympycnus desoutteri ... I can post that up later too.

Can Sympycnus desoutteri be ruled out here at least?
This does seem to have longer hind legs than the other observation.

I see C.scambus is present at this latitude in Norway.
Are there others from genus or subfamily to rule out ?
Is the visible coloration / leg setae, etc... potentially correct for C.armatus?

Many thanks!