Slime Mould - Reticularia lycoperdon
On seeing it on a post by the local shops I thought it was a fungus, but apparently it’s a mould.
According to Naturespotuk it starts off as a white globular mass about the size of half a golf ball, (this one is tennis ball sized) and then develops a silver-grey papery skin beneath which the brown spores develop. When the seeds have been dispersed all they leave behind is a faint brown 'spore print' patch on the tree bark.
Not the most attractive of nature’s gifts but vaguely interesting.