Many thanks to Howard Bentley (former chair of Dipterists Forum) who replied to me via email to suggest genus Hydrotaea and give me some pointers. With his help I've returned to the keys, and it seems to key cleanly to Hydrotaea militaris. There is a lovely mirror-like shiny patch on the upper parafacial that I couldn't get to show up on the photos.
Many thanks to Howard Bentley (former chair of Dipterists Forum) who replied to me via email to suggest genus Hydrotaea and give me some pointers. With his help I've returned to the keys, and it seems to key cleanly to Hydrotaea militaris. There is a lovely mirror-like shiny patch on the upper parafacial that I couldn't get to show up on the photos.