Grey/green Foliose lichen on ash tree
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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I am finding Lichen ID very difficult. Have seen a lot of the lichen in your photo on rotting wood, but have had been unable to distinguish it. Flavoparmelia caperata looks to be a much paler colour (I looked on the Natural history museum guide to lichens on twigs and the encyclopaedia of life) Still, I cant offer an alternative as to what it is.
Lat/Lng: 53.555077281732, -2.2790193557739
OS grid ref: SD816065
Roughly here in the field. Not exact measurement