Luke Hanrahan went to meet Eduard Payne and Rasheed Rollins, co-founders of the YouthDirect website.
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Tencho Hristov ignored warnings and got trapped between barriers at Roydon in Essex.
The 6km bike track will include 130m of the South Circular near Kew Bridge.
The unfortunate officer was taken to a central London hospital for treatment and his condition is not life-threatening.