[1] Queen Mary Univ London, Dept Phys, London E1 4NS, England
[2] Univ Birmingham, Sch Phys & Astron, Birmingham B15 2TT, W Midlands, England
[3] Univ Heidelberg, Kirchhoff Inst Phys, D-69120 Heidelberg, Germany
[4] Johannes Gutenberg Univ Mainz, Inst Phys, D-55099 Mainz, Germany
[5] STFC, Rutherford Appleton Lab, Didcot OX11 0QX, Oxon, England
[6] Stockholm Univ, SE-10691 Stockholm, Sweden