1. Cardiff School of Mathematics, Cardiff University, Cardiff, UK;
2. Department of Mathematics, University of Castilla-La Mancha, Ciudad Real, Spain;
3. Green Templeton College, University of Oxford, Oxford, UK;
4. Wolfson Centre for Mathematical Biology, Mathematical Institute, University of Oxford, Oxford, UK;
5. Gray Laboratory, CRUK/MRC Oxford Institute for Radiation Oncology, University of Oxford, Oxford, UK