Greg took a training session at the Eldon Square shopping centre as part of the launch event.
He won gold at London 2012 long jump and says now it's his turn to inspire others.
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The union claims the company is trying to push through a package which would threaten jobs, working conditions and safety.
The mum-of-two wrongly claimed income support, housing benefit and child tax credits between 2010 and 2014.
A 17-year-old boy was pronounced dead by paramedics at the scene while two other men were taken to hospital.