A purplish-grey crust fungus, probably Peniophora quercina, growing on a fallen oak twig.
I took this twig home with me and, with an elastic band, strapped a microscope slide over the fungus to catch any spores it released. I then placed it slide downwards in a small plastic tub, added a couple of drops of water to keep it damp, and left it sealed overnight. On the following day there was a faint reddish film of spores on the slide. I added a drop of water and then a cover-slip. The microscope revealed the spores to be shaped like slightly bent sausages, which is consistent with the 'curved cylindric, 8-12 x 3-4um' given for the spores of P. quercina. Here they are at x600 magnification:
The original specimen was taken from Whiteknights Park, Reading, UK, on 2012-12-06.