Arthonia parietinaria

Parasitic on Xanthoria parietina, developing clusters of apothecia on the thallus, probably frequent and widespread the lowlands. See http://fungi.myspecies.info/all-fungi/arthonia-parietinaria, https://www.britishlichensociety.org.uk/lichenicolous-fungi and https://www.britishlichensociety.org.uk/sites/default/files/Arthoniacea…