This is either yellow brain fungus (Tremella mesenterica) or witches’ butter (Tremella aurantia); even Wikipedia seems confused about it.
Our Tree Officer, Rich Murphy, found it in the park growing on the ash trees at Fiveways; he says we probably won’t be able to tell which it is without a microscope.
Both T. mesenterica and T. aurantia are parasitic. They grow on the hyphae of wood decay fungi, usually on dead branches that are still attached to the tree.
We are told they are edible but, as usual, our advice is never collect or eat wild fungi without the counsel of an expert.
Pictures by DKG
Another of the park’s fungi: