What I think is a young form of Trametes versicolor. Close-up you can make out the pores on the underside of the developing brackets:
I took the following specimen:
This shows about 3-4 pores per mm.
The specimen yielded the following spores (magnificiation x600, field width 86um):
These spores are about 5.5 x 1.5um. This agrees well with the 5.5-6 x 1.5-2um for Trametes versicolor given at RogersMushrooms.
Sixteen days later the brackets were more fully developed:
First two photos and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2013-12-14. The last photo was taken at the same place on 2013-12-30.