Caloplaca asserigena. This species is best looked for on the stems of Calluna where the characteristically darkish, rust red apothecia indicate it. The thallus is unobtrusive and grey. This photograph was taken on wooden fencing at Dunstaffnage in Western Scotland during a BLS field meeting when it was identified by Brian Coppins. It tends to have a western distribution, but with such a specialised usual  habitat, it is hard to be sure of its abundance.