Division of Public Health Medicine and Epidemiology, School of Community Health Sciences, University of Nottingham Medical School, NottinghamN J JamesDepartment of Family Planning, Nottingham Community Trust, Victoria Health Centre, NottinghamS HughesDepartment of Genitourinary Medicine, Nottingham City Hospital NHS Trust, NottinghamI Ahmed-JushufDepartment of Public Health, Nottingham Health AuthorityR C B SlackCorrespondence to: Dr N James, Division of Public Health Medicine and Epidemiology, School of Community Health Sciences, University of Nottingham Medical School, Clifton Boulevard, Nottingham NG7 2UH.Accepted for publication 26 March 1999