Have you read Alan Garner's collection of essays "The Voice That Thunders"? The second you said this -" I have long wanted to use the "non-myth" of the Green Man with the idea of the natural work v modern life" - made me think instantly of Alan Garner. If you haven't already, I'd read his novels Strandloper and Thursbitch and possibly the Stone Book, but definitely he aforementioned essays.

It sounds from some of the things you mention (escape and parallel worlds) that you might already have done so though...