Declaration of originality

The material in this thesis is entirely the results of my own independent research under the supervision of Professor C. J. van Rijsbergen and Dr M. Lalmas, and is not the outcome of any collaborative work. All published or unpublished material used in this thesis has been given full acknowledgement. Permission to copy Permission to copy without fee all or part of this thesis is granted provided that the copies are not made or distributed for direct commercial advantage, and that the name of the author, the title of the thesis, and its date of submission are clearly visible on the copy. 2 Acknowledgements A PhD thesis has one name on the front. However behind this one person, who takes all the credit, are dozens of other people; cajoling, encouraging, criticising, stimulating and generally making sure that several years of alternating anguish and excitement are eventually transformed into several hundred pages of text. Many people contributed to this thesis, directly or indirectly, either by discussing the ideas contained within, reading huge chunks, o