A small, dark green, brown or blackish liverwort.  Leaves c.1mm, with two rounded and blunt lobes and a V shaped notch between.  The perianths are inflated

There are several species with similar leaves and  of similar small size - e.g.  Leiocolea turbinata, Cladopodiella fluitans. 

If possible, photograph perianths as well as details of leaves and stems; note habitat

damp acidic places - heaths, bogs, etc. - on ground, e.g. peat, sand, gravel or amongst scree

https://www.britishbryologicalsociety.org.uk/learning/species-finder/gymnocolea-inflata/