A polypore fungus, probably Gloeoporus dichrous on a fallen branch from a broadleaf tree.
The lower surface shows about 6 pores per mm:
The upper surface is white and slightly hairy:
Under the microscope (x600, image width 86um), the hyphae show clamp connections:
The spores (also at x600, image width 86um) are allantoid (sausage-shaped) and about 3.3 x 0.7um (this is a the lower end of the 3.5-5.5 x 0.7-1.5um for Gloeoporus dichrous given at MushroomExpert.com):
First photo and sample taken in the Wilderness, Whiteknights Park, Reading, UK on 2013-02-27.