1. ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK
2. Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
3. Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK
4. Institut Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France