Academic public health jobs would be advertised places like www.findauniversityjob.com and jobs.ac.uk
Practitioner jobs may require you to have proper accreditation with the Faculty of Public Health (http://www.fph.org.uk/). If your PG Cert is an academic qualification rather than a practice based training qualification, you may wish to consider applying for their (paid) training scheme.
The Health Protection Agency might be another good place to look, especially if you have an interest in communicable disease: http://www.hpa.org.uk/
Other than that, maybe look at more general sites like The Guardian and local authority websites. My suspicion is that you would be looking at entry level posts as more senior posts probably require higher qualifications or previous experience.