Have you checked John Martyn's websites? One, which you can get to by typing in gets you to the John Martyn connection which has a links area hooking you up to guitar things. According to that, there's information on tunings for every song John has written. However I must say, that when I've tried to access stuff for individual songs, that it doesn't work on my computer, so you may be setting yourself up for frustration. There is also a which is a different site. There the guitar stuff is for a few songs from Martyn's acoustic guitar hero phase (Head & Heart, Bless the Weather, Over the Hill) but doesn't address the question you have. I think on at least one of the guitar sites there's an email address to someone who has delved deeply into John Martyn's guitar style and may be able to help you better. Good Luck, I like the song too.