Acknowledgements

This work is partly supported by the UK-EPSRC grant GR/S63267. The first author has also been supported by a College of Science and Engineering scholarship at the University of Edinburgh. We are very grateful to Robert Muetzelfeldt for helpful discussions and assistance in the research reported, whilst taking the full responsibility of the views expressed here. Thanks also go to the anonymous referees for their constructive comments which are very useful in revising the earlier version of this paper.