[1] Univ Oxford, Dept Engn Sci, Parks Rd, Oxford OX1 3PJ, England
[2] DPI, POB 902, NL-5600 AX Eindhoven, Netherlands
[3] ESRF European Synchrotron, CS 40220, F-38043 Grenoble 9, France
[4] Univ Manchester, Henry Royce Inst Adv Mat, Dept Mat, Manchester M13 9PL, England