Centre for Mathematical Biology, Mathematical Institute, Oxford, United Kingdom. murrayp@maths.ox.ac.uk|Centre for Mathematical Biology, Mathematical Institute, Oxford, United Kingdom. murrayp@maths.ox.ac.uk|Centre for Mathematical Biology, Mathematical Institute, Oxford, United Kingdom. murrayp@maths.ox.ac.uk|Centre for Mathematical Biology, Mathematical Institute, Oxford, United Kingdom. murrayp@maths.ox.ac.uk|Centre for Mathematical Biology, Mathematical Institute, Oxford, United Kingdom. murrayp@maths.ox.ac.uk