I think you know well that there is a decent amount of case law on this - so CONSIDERABLY more than once every two decades.
I'm not a lawyer and have never studied law so no I don't know the number of cases on this.
What is even more disturbing than this new law however is this case http://www.independent.co.uk/news/uk/crime/blogger-wrote-of-murdering-girls-aloud-949711.html where someone is being prosecuted for writing a story, Hitler would be proud I'm sure. Will be interesting to see if the jury nullify this.