Careful, don't aggravate John McDonnell even when he can't answer a simple question

John McDonnell showing his mastery of his brief there. Is he incapable of answering a simple question or does he just not want to answer the question.

If I were the interviewer I'd be a little worried as towards the end of that interview John 'knee-capping might help to change their minds' McDonnell sounded a little irritated.