There's a faultline that lies beneath much of the current turmoil in the Middle East, and it runs between Riyadh and Tehran. Sunni Saudi Arabia and Shia Iran are locked in a series of conflicts by proxy. It's a dangerous and costly struggle for regional supremacy that weaves from Lebanon to Yemen by way of Syria and Iraq. Stephen Sackur talks to Mohammad Marandi in Tehran and Jamal Khashoggi in Jeddah. Is there any way to take the heat out of the Saudi-Iranian confrontation?