In memory of John Sample (25th November 1970 - 29th January 2007). He was a very kind and generous person who went out of his way to help others. He loved learning and sharing ideas. He will be sadly missed by his family and friends.\ud \ud This paper is also available from the departmental website at http://www.le.ac.uk/mc/research/papers.htm
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.