Model-based geostatistics - Discussion by Webster, R, Lawson, AB, Glasbey, C, Horgan, G, Elston, D, Host, G, Mugglestone, M, Kenward, MG, Kent, J, Stein, A, Clifford, P, Ledford, A, Marriott, P, Aitkin, M, Atkinson, A, Boskov, M, Kelsall, J, Wakefield, J, Bowman, A, Casson, E, Cressie, N, Denison, D, Mallick, B, Dixon, P, Scott, M