Yellow Brain Fungus

I think this is Yellow Brain Fungus (Tremella mesenterica). It was growing on a fallen branch in the Serpentine Walk. It is a very bright yellow fungi that grows on dead and decaying fallen branches of deciduous trees, especially oak, beech, and hazel.  It develops folds, which do look a bit like the structure of a brain, which gives it its name. It is also sometimes referred to as Witch's Butter. It is most noticeable after wet weather.

Another example a little further on in the Serpentine Walk.