It appears to be Plicatura crispa, a fungus with a northern distribution, mostly reported in Scotland. Details can be found at this web site. There's a distribution map here.
The dense clusters of fungi were arranged in tiers and mostly concentrated around burrs on the trunk of the tree, but clearly spreading outwards from there.
Each individual fructification is quite small - no larger than a thumbnail - but there are often over 100 in a cluster.
The upper surface is brownish-buff, paler towards the edge, and the undersurface is white.
From below ....
... the gills have a wrinkly appearance.
I need to go back and collect a specimen now, for a closer look.