that doesn't have a definitive answer. Eight years ago there was a high-profile case when Internet Service Provider Demon was sued by a Physicist for refusing to remove defamatory statements about him from an online Newsgroup discussion. The statements were posted by a forum member, but Demon was sued, and settled out of court by paying the man £200,000 in damages.
There have been other cases, and although some of them have been elsewhere in Europe there's no room for complacency.
My advice is that if you intend to run an online discussion forum you run it as a tight ship - check postings regularly, and delete anything that is in your opinion defamatory. Don't be bullied by third parties, but don't ignore anyone who complains - make it quite clear to your forum users that you will not tolerate defamatory statements or sweeping allegations that have no basis in fact.
Read as much as you can on the libel laws, and remember that it's not just possible to libel an individual - you can libel a group of people as well. If one of your forum users says, for example, that "All online forum editors are criminals" you might find yourself standing alongside him in a libel court, facing an action brought by the National Association of Forum Editors. If, on the other hand the same person posted saying that "I believe some online forum editors are criminals" it would be OK.
The rule is, if in doubt, delete - it's better to explain your action to an irritated forum user than to find yourself facing a libel action.