What is the UK law on 'anonymous' libels published online?

If I write to a UK newspaper the paper's lawyer will check it out for anything actionable because if the paper published the letter they would become become liable for any libel it contained. But I can comment on Yahoo news stories "anonymously" and allege all sorts of things against named or easily identifiable people. We had one on Yahoo today alleging that a girl who had her jail sentence quashed had slept with the judge to achieve that, Now that is libel in anybody's book and in the UK it could be a criminal libel because there is a deliberate intent to do harm to two reputations.
How is this possible? How can 'trolls' get away with it?
If a libelled person demanded that Yahoo compensate them for publishing the libel what would Yahoo's defence be? Could Yahoo be forced to reveal the troll's identity?
A lot of questions - sorry!

The Arbiter of common sense2011-08-19T17:02:18Z

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Actually, not all newspapers will censor letters and other comments. They protect themselves by stating that "All opinions and letters are the opinion of the write, and do not represent the view of the paper" In the case of online, it is the same thing, nobody is responsible except the person writing it. If there is active libel, a subpoena can be used to identify the person posting, but it's rare.