Though I haven’t been able to verify its identification, I’m fairly sure today’s fungus is Phellinus pomaceus.

It’s a hard, woody bracket fungus that grows on Prunus tree species – in this case, it’s growing on Blackthorn (Prunus spinosa).

The NBN (National Biodiversity Network Trust) Atlas entry for this species (which also includes a map showing where in Britain the fungus has been recorded) says ‘It is not aggressively pathogenic but can cause considerable decay in trees suffering from other stress factors’, so you wouldn’t want to find it in a commercial fruit orchard. In my case, the fungi were only showing on two adjacent trees in a large copse of Blackthorn, and the trees looked quite elderly, so I don’t think it’s causing a problem.