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Green Wood Cup Chlorosplenium aeruginascens: the fruiting bodies are seen a lot less often than the green stain they impart to the wood.
If the stain continues inside the wood, that is what it will be: it could be a superficial lichen though, so I've not offered this as a revision.
Lat/Lng: 50.9, -1.7
OS grid ref: SU2616