Identified by Else Vellinga. My understanding from her is that Tremella mesenterica does not parasitize Stereum as I had heard, but rather Peniophora, another wood rotter. The Stereum parasite is Tremella aurantia.
Here are some literature references provided by Else Vellinga. Peter Roberts from Kew Gardens has written several articles on Tremella in Britain:
1995: British Tremella species I. Tremella aurantia and T. mesenterica. Mycologist 9: 110-114.
1999. British Tremella species II. Tremella encephala, T. steidleri, and T. foliacea. Mycologist 13: 127-131.
2001. British Tremella species III. Tremella callunicola sp. nov., T. invasa, T. sarniensis sp. nov., T. simplex, & T. versicolor. Mycologist 15: 146-150.
2007. British Tremella species IV. Tremella obscura, T. penetrans, T. giraffa & T. polyporina. Field Mycology 8: 127-133.
the first one treats the species we see: Tremella mesenterica parasitizes Peniophora species, T. aurantia is growing on Stereum.
Looks like I’m going to have learn to recognize Peniophora species!