Specifically for the UK - what is the law on libel and slander by
people making postings on web sites which belong to others, such as
Opinions web sites, review web sites and general discussion boards. Is
there any amount of conditions of use which enables the site owner to
avoid legal action by the people who are libelled or slandered. Anyone
got any creative ideas on the subject?

Can you direct me to any high profile cases? 

Is removing the offending posting enough or would the web site owner
still be liable for legal action?

I expect that the key is a terms of service agreed to by persons
registering to post on the site, plus a disclaimer of responsibility
for posters' message contents.

You might look at Google's Terms of Service below.