Arthonia radiata (Pers.) Ach.
Immersed, pale grey to whitish thallus with black apothecia, variable in shape but usually irregularly stellate. Found on nutrient rich smooth bark and twigs, often on Ash (Fraxinus excelsior)
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