THE TUMBLER (LP)Following an inauspicious first release for Island, John Martyn's unique brand of acoustic music began to take some sort of shape with the Al Stewart produced The Tumbler. Martyn, whose debut was a solo acoustic offering, here employs a second guitar, bass and, most notably, jazz flautist Harold McNair to create his blend of folk, blues and jazz. While his songs show only marginal development from London Conversation, it's his arrangements and,