Lone-spot Winter Gnat Trichocera regelationis
The dark wing spot combined with the uniform unbanded abdomen should have been sufficient to identify this, but I keyed it out properly (using Stubbs) before realising that.
male Lone-spot Winter Gnat Trichocera regelationis showing wing, abdomen from above, tarsal claws and genitalia (various views), Wendling Beck Project (Norfolk, UK), 21st March 2024
female Lone-spot Winter Gnat Trichocera regelationis showing wing, abdomen from above and cerci from side, Wendling Beck Project (Norfolk, UK), 16th April 2026