Lecidella elaeochroma f. elaeochroma (Ach.) M. Choisy
Common on well-lit smooth bark, especially twigs and small branches, also on worked timber.
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species