A pale buff-colour crust fungus, probably Stereum rugosum, growing on a birch tree (Betula sp).
I took this specimen home with me:
It yielded these spores (magnification x600, field width 86um):
These are about 10.5 x 4um which agrees with the 9–13 x 3.5–5um given for Stereum rugosum at RogersMushrooms.com. The spores of most other Stereum sp seem to be noticeably smaller. The above specimen was rather old dessicated which probably explains why it didn't bleed when cut.
First photo and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2014-02-11