I took a small sample from the edge to examine with a magnifier at home. It showed 2 to 3 pores per mm on the underside. The pore tubes were about 5mm long. Under x600 on the microscope I was unable to find any clamp connections. These details are consistent with the fungus being H. annosum.
[Note added 2012-12-24: I managed to get some spores from a small sample of this fungus. These spores were about 10 x 8um. This is too large for H. annosum. It is more likely a Ganoderma sp, maybe Ganoderma resinaceum.]
Photos taken in the Wilderness, Whiteknights Park, Reading, UK, on 2012-11-22.