A toothed crust fungus, probably Steccherinum ochraceum growing on an oak twig (Quercus sp). The scale is given by the millimetre rule at the bottom.
The following image shows the spores at x600 magnification. They are about 3 x 2um (the image is about 86um wide) which agrees reasonably well with the 3.5-5 x 2-2.5um given at MushroomExpert.com.
Specimen collected in the Wilderness, Whiteknights Park, Reading, UK, on 2012-12-23.