Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
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At the time I made this observation I thought that the tree was a cherry, but is now (27 March) in bud and it looks like a whitebeam, Sorbus aria. Its still alive, though the trunk is fairly rotten as you can see.
University of Edinburgh and Biodiversity Observatory (OU)
Later in the year this tree turned out to be a horse chestnut and on 29th Jan 2012 I was passing and noticed there was a new crop of oyster mushrooms on the trunk, though unfortunately past their best.
Lat/Lng: 52.011235, -0.7094966
OS grid ref: SP886355