A rather lettuce-like lichen which I think might be a greenish form of Xanthoria parietina.
I took the following specimen:
This yielded the following spores (magnification x600, field width 86um):
These are about 12.5 x 7um which agrees well with the range 12-16 x 5-9um given for Xanthoria parietina at the Consortium of North American Lichen Herbaria.
First photo and specimen taken in Whiteknights Park, Reading, UK, on 2014-01-11.