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Film-maker Robb Leech found his step-brother Richard, now called Salahuddin, would only shake hands using his "dirty" left hand since becoming an Islamist.

The pair were brought up in the English seaside town of Weymouth, Dorset, but Richard converted to Islam five years ago.

Robb spent time with him and other young converts, including 17-year-old Ben, who explained some of their beliefs.

READ MORE: Why did my brother become an extreme Islamist?

My Brother the Islamist is on BBC Three 4 April, 2100 BST.