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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This looks like an Artist's Bracket (Ganoderma applanatum). Confirm this by scratching the white area with a sharp item (key etc) the mark will turn instantly brown, hence its name!
Unfortunately G australe does exactly the same. See http://www.ispot.org.uk/node/89279?nav=users_observations
Thanks for info, I forgot about the southern bracket.
Lat/Lng: 50.71976, -1.8783
OS grid ref: SZ086911