A white porecrust fungus, probably Schizopora paradoxa, growing on a dead branch of an oak tree (Quercus sp). The above section was on the side of the branch and shows the pores side-on, while the following section was on the underside and shows the pores end-on:
This is the spores at x600 under the microscope (prepared in the way given here):
The width of the image at x600 is about 86um (determined by photographing a millimetre ruler at x60) so the spores are about 6 x 3um which is consistent with the '4–6 x 3–3.5um' given at Roger's Mushrooms.
The hyphae show clamp connections, see one at the top left of the following image (x600):
First two photos taken in Whiteknights Park, Reading, UK, on 2012-12-18.