1. Hopkinson Laboratory, Cambridge University Engineering Department, Cambridge CB2 1PZ, UK e-mail: jby@eng.cam.ac.uk
2. E.ON UK plc, Westwood Way, Coventry CV4 8LG, UK
3. Technology Centre, E.ON New Build & Technology Ltd., Ratcliffe-on-Soar, Nottinghamshire NG11 0EE, UK