Drymus pumilio is a particularly small, dull brown species, length 2.5-3mm. Small tooth on front femur; tibiae with hairs shorter than width of tibia; pronotum black anteriorly, brown posteriorly; forewings usually at least as long as the abdomen. 

Other Lygaeids, especially Stygnocoris fuligineus, but this has a uniform series of fine erect spines at right angles to the
ventral face of the femur, c.f. a tooth.

Detailed photos and measurements required. 

Chalk grassland, but also from wet grasslands and fens.

All year. 

Found locally across southern and central England and parts of south Wales.  (Nationally scarce NS (Nb) - ref. UK Heteroptera checklist 2021)

Not often recorded. 

https://www.britishbugs.org.uk/heteroptera/Lygaeidae/Drymus_pumilio.html