On the underside of a fallen birch trunk: a yellowish crust fungus, probably Stereum gausapatum.
It bleeds red liquid hen its surface is cut:
I took the following specimen home to look for spores:
And this it what it gave (x600 magnification, field width about 86um):
These spores are about 8 x 3.5um. This is a bit smaller than the 9-13 x 3.5 - 5um given for Sterum rugosum at Rogers Mushrooms but nicely matches the 7-8 x 3-3.5um for S. gausapatum given by the same source.
Apparently, S. sanguinolentum has spores the same size as gausapatum but that species grows only on conifers, not on birch.
First 2 photos and specimen taken in the Wilderness, Whiteknights Park. Reading, UK, on 2013-12-21.