A grey crust fungus, probably Peniophora cinerea, growing on a fallen beech branch (Fagus sp).
I took the following specimen home with me:
It yielded the following spores (magnification x600, field width 86um):
These are about 9.5 x 3.5um which is at the large end of the range 7-9(-10) x 2.5-3.5um given for Peniophora cinerea given here at MycoBank.
First photo and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2014-03-02.