I wondered if this might be a Large Rose Sawfly but I don't think it is. The head's too small. Maybe it's some sort of Gnat.
It was found in the back garden (Dartmouth, Devon) sitting on an arbutus leaf and didn't seem to mind being snapped.
I would be grateful if someone could point me in the right direction please.
That's a fungus gnat in the genus Sciara.