from a youtube video: Tom Wright (bishop of Durham) speaking:

"(Unlike the other three gospels) John's gospel has not been shaped by the church telling the story so often that it fell into an anecdote form . . . I think John's gospel grows out of a memory of a man who has been praying over this stuff again and again and again. I can't prove that but it makes quite a lot of sense to me. Within that, of course, his life of prayer and preaching and devotion and holiness has coloured the way he says it but I think that John's gospel goes back to Source rather than to sources . . . . . ."

and in another video:
"and what is this trinity? It is the Jewish monotheism of the Exodus stories seen in the light of Jesus and the Spirit."