Jody Barnard informed me that he has a new article out:
Ronald Williamson and the Background of Hebrews." Expository Times 124 (July 2013): 469–79.
"In 1976 Ronald Williamson published a brief article in the Expository Times
suggesting that an early form of Merkavah mysticism best explains some of the
distinctive features of Hebrews. The purpose of the present article is to
revisit this suggestion in the light of subsequent research, and to set forth a
revised and more robust version."