Wrinkled crust fungus, Phlebia radiata, growing on a fallen branch, probably from an oak tree.
I took the following specimen home with me:
And it yielded the following spores (magnification x600, field width 86um):
These spores are about 4 x 1.5um which is at the lower end of the range 4-4.5 x 1.5-2um given for Phlebia radiata here.
First photo and specimen taken in Whiteknights Park, Reading, UK, on 2013-12-24.