It charts his attempt to reconnect with his stepbrother, Richard Dart, who had in the summer of 2009 converted to Islam under Anjem Choudary, the leader of now proscribed Islamist group Islam4UK.
Leech began filming with Dart just a few months after the conversion and continued up until the moment he left for his first Hajj, late in 2010.
The film presents a rare insight into the world of Islamic fundamentalism, the young men caught up in it, and the devastating effect it has on families.
First aired on BBC Three in April 2011, My Brother the Islamist has since been broadcast across the world, in countries including Australia, Austria, Canada, Germany, Japan, New Zealand and the USA.
The documentary also won Robb Leech Best Debut Director - Edinburgh Television Festival (2011).