J. J. Bissell, Department of Mathematical Sciences, University of Durham, UK. C. C. S. Caiado, Department of Mathematical Sciences, University of Durham, UK. S. E. Curtis, Institute of Hazard, Risk and Resilience, University of Durham, UK. M. Goldstein, Department of Mathematical Sciences, University of Durham, UK. Brian Straughan, Department of Mathematical Sciences, University of Durham, UK.