The fungus that Clive Knight photographed on the ash tree at Fiveways is Inonotus hispidus commonly known as shaggy bracket, shaggy polypore or velvet fungus. Our Tree Officer, Rich Murphy, identified it for us.
It is usually visible as a solitary fungus on the trunk and branches of broad-leaved trees, especially ash and apple. It is fairly common and widespread, and the fruiting bodies appear from late summer right through into winter. The two photographs below, taken less than a week apart, show how rapidly this species can grow; the brackets can be more than 30cm across when they are mature.
This is not particularly good news for the tree. Inonotus hispidus is classed as a white rot decay fungus and infected trees are weakened over the years, which can result in trunks or branches breaking and falling in stormy weather.