According to the piece, the mild mannered son of teachers from Weymouth, who had dreamed of becoming a carpenter was now one of the extremist's most devoted followers and was spending his days calling for strict Sharia law to be imposed in Britain.
Unable to believe what he was seeing, Robb, who had been extremely close to his stepbrother when they were teenagers contacted him and they arranged to meet him in London.
What he found disturbed him immensely and led to a BBC documentary entitled, My Brother The Islamist, which charted Dart's transformation from a middle class teenager to a hate filled extremist.
Describing his first meeting with his newly radicalised stepbrother, Robb said: "It was worse than I could have hoped for. I met him off the tube and he didn't say hello or how are you, he just tried to convert me for five hours straight. Some of the first words he said were "Look at these filthy Kuffar (non-believers). None of this will be around when Sharia law comes in."
Despite his new found contempt for Western culture Dart, who now went under the name Salahuddin Al Britaini, agreed to appear in the documentary, believing it offered the opportunity of Daw'ah [sharing the message of Islam], that is one of the central parts of the faith.
But rather than convincing viewers of the merits of the religion, he spouted vile hatred about his own country and at one point even refused to shake his stepbrother's hand, claiming as a Kuffar, he was unclean.
Dart fell under Choudary's spell while studying carpentry at a college in West London.
He claimed his first meeting with the radical preacher changed his life forever and he gave up drinking alcohol, eating pork and announced he would be happy to fight and die overseas to defeat Western aggression against Islam.
In the documentary Dart is seen on a loud hailer condemning soldiers returning from Afghanistan as "murderers".
With grieving relatives in earshot, he shouted: "You foolish people risk your life for these degenerate rulers - these people who continue to guide you into the hellfire. 'Wake yourself up from your slumber and educate yourself, you foolish people."
During the documentary, Dart also met with a young, enthusiastic Muslim convert named Ben, 17, whose mother was concerned by his new religion.
Explaining why he made the documentary Robb said: "I made this film to try and help my family understand it all.
"The idea that we are all dirty and the only way to escape eternal suffering and the hellfire is to join Rich's group of fundamentalists really bothers me.
"It doesn't matter how charitable, good-willed or selfless you are if you don't conform you will burn."
Shortly after the documentary was screened it was revealed that Dart was living in a £300,000 luxury flat in East London and was claiming thousands of pounds a month in benefits.