9 pages, 8 figures, accepted for publication in MNRAS. A version with
  full resolution figures may be obtained at
  http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.ps.gz (postscript) or at
  http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.pdf (PDF)