A toothed crust fungus, probably Steccherinum ochraceum, growing on a birch twig (Betula sp). The top edge is beginning to curve over to form brackets. I found this twig only a few metres away from the birch stump with probable S.ochraceum brackets that I mentioned here.
The spores at x600 (86um image width), are about 3 x 2um:
Sample taken in the Wilderness, Whiteknights Park, Reading, UK, on 2013-02-02.