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 et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species