Hapalaraea pygmaea
This was caught in a vane trap and identified after Duff's key was published. Actually I didn't immediately get a successful resolution using Duff as at the angle I first looked at the hind legs the fifth segment looked about the same length as the first four, or at least only a very little longer. As it says marginal cases keyed both ways I didn't worry about measuring it precisely and ended up with a dead end. I then attempted it using Hackston's key and reached Hapalaraea pygmaea before going back to Duff and realising the point where I had gone wrong was where I judged the relative length of the hind tarsomeres. Looking back at the hind tarsus I realised that the fifth tarsomere was indeed longer than the first four, by a clearer margin than I had first thought. Keying it that way did indeed end up with Hapalaraea pygmaea using Duff. It was a female though I couldn't find a spermatheca.
female Hapalaraea pygmaea showing head and pronotum (from side and above), head, maxillary palps, pronotum, humerus and hind tarsus, North Elmham (Norfolk, UK), 5th May 2024