1. Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
2. Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
3. School of Physics and Astronomy, University of Southampton, Southampton SO17 1SJ, UK
4. School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
5. CEA-Saclay, F-91191 Gif-sur-Yvette, France
6. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7. ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands
8. Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, The Netherlands
9. Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK