Only just caught up with this one. I would recommend popping this on ASCOfrance. Alain Gardiennet is producing an impressive resource of lichenicolous fungi there. Four species of Sphaerellothecium on that site, but not yours. PLus those are good quality images.Chris Johnson wrote: ↑Mon Jun 18, 2018 7:52 amSphaerellothecium minutum.
Smith, et al – The Lichens of Great Britain and Ireland note: 'The most common lichenicolous fungus on S. fragilis is the small ‘pyromycete’ Sphaerellothecium minutum Hafellner ..."
I'm grateful to Paul Cannon who advised it was what I have, told me what to look for, and provided literature. He will be credited with the identification. A sample has gone to Kew. Apparently the third British record.