[1] Department of Computing, Imperial College London, 180 Queen's Gate, London SW7 2AZ, United Kingdom
[2] Martyn Thomas Associates, Bath BA2 4QP, United Kingdom
[3] Computer Laboratory, University of Cambridge, Cambridge, United Kingdom
[4] Cambridge University, Cambridge, United Kingdom
[5] University of Birmingham, Birmingham, United Kingdom
[6] Pervasive ICT Research Centre, BT Group Chief Technology Office, British Telecommunications plc, 81 Newgate Street, London EC1A 7AJ, United Kingdom
[7] School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, United Kingdom
[8] ECS, University of Southampton, Southampton, United Kingdom
[9] Department of Computer Science, University of Bath, Bath BA2 7AY, United Kingdom
[10] Department of Computer Science, University of Liverpool, Liverpool L69 7ZF, United Kingdom
[11] Institute for Research in Applicable Computing, Department of Computing and Information Systems, University of Luton, Park Square, Luton LU1 3JU, United Kingdom
[12] Digital Technology Group, Computer Laboratory, Cambridge University, Cambridge, United Kingdom
[13] Department of Informatics, University of Sussex, Brighton, United Kingdom