RAL would like to thank the National Radio Astronomy Observatory, the Istituto di Radioastronomia, Bologna and Alan and Mary Bridle for hospitality during this project. We thank Rick Perley for help with the observations and Matt Lister for a careful reading of the manuscript. We acknowledge travel support from NATO Grant CRG931498. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.