Blue/grey foliose lichen on wood
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Some fungi are very poisonous so a mistaken ID could have serious consequences.
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If you compare your photo with the photos in the link below you'll see its not Hypogymnia physodes as that has hollow lobes that are the same colour on both sides.
Your lichen has a black lower surface but I can't see details clearly enough for an ID.
Hypogymnia physodes: http://www.irishlichens.ie/pages-lichen/l-09.html
Thank you for the feedback. I can see what you mean from the picture on the weblink you provide. The picture in the book I used was identical to my photograph, but on further investigation using the detail in the original photograph, and in light of your feedback I now believe it to be Parmotrema perlatum. I shall amend the entry accordingly and await further feedback.
Lat/Lng: 52.3, -1.9
OS grid ref: SP1074