I found this tiny sample among a collection from a stony forestry track margin at Cefn Mably Wood, NE Cardiff, last weekend.
The curved, concave leaves suggest Hygrohypnum, and the inflated alar cells also seem to fit with it being a member of this genus.
The habitat was dry and probably calcareous (Dicranella varia was an associate), suggesting H. luridum as the most likely species. However, there is no trace of a nerve, which I think is always a feature of this species. The other Hygrohypnum seem unlikely given the habitat, so perhaps I'm barking up the wrong tree and it's not Hygrohypnum at all. Any suggestions welcome, thanks.