Anyone who repeats allegations can also be sued. This is important. Seeing something written somewhere else doesn't mean it is true. Repeating allegations without making sure they are true is a very good way to get yourself knee deep in litigation.
For example, say you wrote an email to the top 200 managers and governors of the BBC about Mr Dyke never paying his licence. That email is leaked to Magazine X who print it without making sure it is true.
Although the mail's already been sent and read by all, by repeating the allegation they too are committing the same libel. 'Repetition is no defence' say lawyers. Because it isn't.
http://www.urban75.org/info/libel.html