[1] Univ Oxford, Denys Wilkinson Lab, Keble Rd, Oxford OX1 3RH, England
[2] Univ Bath, Bath BA2 7AY, Avon, England
[3] Univ Bristol, HH Wills Phys Lab, Tyndall Ave, Bristol BS8 1TL, Avon, England
[4] European Org Nucl Res CERN, CH-1211 Geneva 23, Switzerland
[5] Univ Edinburgh, Sch Phys & Astron, James Clerk Maxwell Bldg, Edinburgh EH9 3FD, Midlothian, Scotland
[6] Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England
[7] Photek Ltd, 26 Castleham Rd, St Leonards On Sea TN38 9NS, E Sussex, England