A medium-sized and very variable Mycena.  The cap is dark brown, greyish or blackish-brown, with a paler margin, and paler when dry; stem and gills are greyish.  The smell is nitrous.  

There are several very similar Mycena found in the same habitat.

microscope/smell

Cheilocystidia clavate; usually smooth and without any projections.  The stem cystidia are irregular and inflated.

This is a difficult species, and we recommend that it is verified by an expert from a specimen before submission to Naturespot.  Microscopic examination will be needed.  Also photograph from top down, in side view and underneath to show gills; note smell, habitat and substrate.

In grass or moss in woodlands, or on stumps or decaying hard- or softwood, 

Autumn.

Widespread and fairly frequent in Britain.

Fairly frequent in Leicestershire and Rutland.