1. STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, and Institute Laue Langevin, 6 Rue Jules Horowitz, F-38042 Grenoble, Cedex 09, France