Caloplaca herbidella. In this photograph, it is on a tree in an avenue in Kolderveen in the Netherlands. The Diploicia canescens to the right of the photograph gives an idea of scale. Note that it is a scurfy, grey thallus which is minutely isidiate, and is best searched for with a handlens. In the infertile state, it is not confined in Britain to ancient woodland, and may be seen on well lit, basic barked trees