CJZ is indebted to Professor M. J. Seaton for his generous hospitality at University College London. CM acknowledges a research assistantship from the SRC. We are very grateful to Dr J. O. Ekberg from the University of Lund;
Sweden;
for useful information in connection with experimental energy levels. The computing was carried out on the IBM 370/165 computer at the SRC Daresbury Laboratory;
Cheshire;