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.
Southwick Country Park Nature Reserve 
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.