A yellow disco fungus, probably Bisporella sufurina.
I took this specimen home with me and from it obtained the following spores:
With magnification x600 the image width is about 86um so these spores are about 8 x 2.5um. This matches the range 8-11 x 2-2.5um given for B. sufurina here and is distinctly smaller than the 9-14 x 3-5um given for the commoner Bisporella citrina here .
First photo and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2013-10-26.