The notorious tale of Sweeney Todd, the demon barber of Fleet Street is quite rightly considered a modern masterpiece of musical theatre. The Stephen Sondheim musical classic is a dark and macabre tale of murder, madness and cannibalism, all held together by a glorious score and just a little humour. The newest production, directed by Joe Wright, is currently wowing audiences at the Birmingham Rep, and last night I went to see it and was frankly astounded by its absolute brilliance and horror.  The show has the power to thrill and horrify an audience at the same time. Benjamin Barnes has just arrived back in London, 15 years after being…