---

Thanks Paul, you're absolutely right. After all these years I realise where I had been going wrong. Chyromyidae have the subcostal vein complete , separate from R1 and ending in the costa. It doesn't appear that way from my specimen, probably because of distortion after drying and the fact it's so small.
Using the key to British Aphaniosoma it would be either socium or melitensis, which are best separated by the structure of tergite 4 of the male. I can see no trace of any projections, although the abdomen is a bit shriveled. Also melitensis is said to have yellow on the scutum & scutellum, which is absent in my specimen. So socium is the most likely candidate.

Thanks again,

Michael