School of Crystallography, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK. a.clegg@mail.cryst.bbk.ac.uk|School of Crystallography, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK. a.clegg@mail.cryst.bbk.ac.uk