1. a Department of Agricultural and Food Economics, School of Agriculture, Policy and Development, The University of Reading, PO Box 237, Reading RG6 6AR, UK. E-mail: R.M.Bennett@reading.ac.uk
2. b Centre for Agricultural Strategy, School of Agriculture, Policy and Development, The University of Reading, Reading RG6 6AR, UK
3. c Land and Water Use Management Unit, Scottish Natural Heritage, 12 Hope Terrace, Edinburgh EH9 2AS, UK