Found in sample taken from salt marsh on Eden Estuary, Fife.
No interactions present.
That's a lovely clear photo, and I agree it's Diptera, but I don't know enough about these to confirm the species or genus, sorry!
Entomologist and biological recorder
This was my first thought when I first saw it. I am just not 100% sure. Did it have a typical way of movement - almost rigid looking and thrashing in a sinusoidal manner? Have a look at the following link for one that I have found.
As you will sse it has a much more slender build and this is why am not sure.
Lat/Lng: 56.3599949, -2.8596055
OS grid ref: NO469189