"Habitat: On soil (often calcareous) in deciduous woodland. Often
associated with Quercus spp., but also reported with Fagus and Fraxinus.
Notes: This is a small but distinctive species, possibly genuinely rare
though widespread in Britain. Known from England (Bedfordshire,
Berkshire, Herefordshire, Hertfordshire, Oxfordshire, Surrey, West Kent
and West Sussex) and a single collection from Scotland (Easterness)."
Ref: BC
More info and a modified key supplied by Ken in forum
Ref:
http://tinyurl.com/4nl4by