Beefsteak Fungus.

I spent most of the afternoon collecting the rubbish found yesterday abandoned by a band of merry campers; more of it has accumulated overnight. As chance would have it, I heard an unattributed quote on the radio today, "God put me on Earth to help others, God only knows why He put the others here."
On one of my many journeys humping the stuff the half mile to the car I met a young couple who thought I was removing stuff that they'd collected a couple of days ago; on discovering it was a different load they asked if I could spare a few plastic bags. I now have two potential members of the Friends of Roslin Glen and, with subs at £1 p.a, it should be easy enough to secure them.

I then met a couple of ladies who'd come to commune with the Sweet Chestnut trees, they were caressing and cuddling them, I felt quite jealous. They asked me to take some photos of them using their I-pad; they were rather fussy but I did leave them with plenty to choose from. They claimed that the two trees were the same age and that the older looking one was the male portion of a happy couple. It would make sense that the pair were planted at the same time but equally, it's possible that one died after a hundred years or so and was replaced. They went on to say that the males can be identified because their trunks bulge at the base while the females bulge at the crown. (Wikipedia disagrees with most aspects of these theories.)
I feel that some research is needed

Looking at my Ladybird book of mushrooms, the only one to have the right habitat and to look anything like these is the beefsteak (Fistulina hepatica), it's a bit less than the smallest quoted size but it's only appeared recently so may still have a little growing to do.
Note: The right habitat is low down on the trunk of an aged and decrepit sweet chestnut tree.