Opegrapha atra Pers.
Widespread but easily confused with other graphid lichens. Thallus creamy but can look quite green with algae present. Crowded lirellate apothecia
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