Jimmy Carr, a comedian who has more than once got into hot water over jokes that some find offensive, does a very strange thing for the encore of his latest show, Laughter Therapy - he gives a lecture cum homily on the limits of offensiveness, and how anything is permissible if the audience allows it. “I know my jokes are cruel and brutal and unacceptable,” he says. “But they have only one purpose - to make you laugh.”