Former jihadi Aimen Dean gives a unique insight into the workings of Islamic State. Dean left school in Saudi Arabia to fight jihad in Bosnia in the 1990s. But with the rise of al Qaeda he became disillusioned with his comrades' drift towards terrorism. He joined al Qaeda - but working undercover for the British government. Dean has recently spoken publicly against the jihadist movement but he retains a deep network of contacts within it. Despite Dean's defection, IS supporters still debate with him. Through those discussions, Dean has gained a profound understanding of the ideology and organisational networks behind IS.

Reporter: Peter Marshall