These are densely packed crustose lichens, on the bark of a young birch tree in Sheepfield Copse. Groups of lichen species are often consistently associated together, forming recognisable communities. It is probable this is a community, containing several species of Arthonia, that grows on smooth barked trees.
Before we moved to Wiltshire we lived on the coast in Kent. We never saw lichens so it was a surprise to find them growing everwhere in this area, on trees, paving etc. Is itsomething to do with the air quality perhaps?
They are very mysterious; I am having trouble finding an expert in Wiltshire who will help us identify our lichens.