Green lichen on oak.
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If you try this identification key from the Fungi and Lichen forum you should be able to identify this common yellow or greenish-yellow lichen:
Thanks Jenny, but unfortunately the link comes up as "You are not authorized to access this page" :(
That's odd, I've just checked the link and didn't have a problem.
Here's the link to the Natural History Museum's 'Guide to Lichens on Twigs' key:
Lat/Lng: 53.2511413, -3.131467
OS grid ref: SJ246732