The authors wish to thank Prof. S. T. Thoroddsen and M.-J. Thoraval for some helpful discussions, and the reviewers for useful comments. This publication was based on work supported in part by Award No. KUK-C1-013-04, made by King Abdullah University of Science and Technology (KAUST). M.R.M. would like to acknowledge the EPSRC for financial support via a studentship. J.R.O. was in receipt of a Leverhulme Emeritus Fellowship.