Paul Simon's Graceland album is one of his greatest achievements - a brilliant fusion of African rhythms and western pop which became a global phenomenon. It also proved hugely controversial, as Simon broke the UN-backed cultural boycott of a country still under the grip of apartheid.Joe Berlinger's film captures Simon's return to South Africa 25 years on and contrasts the value of individual artistic expression versus collective political action as instruments of change. Did Paul Simon's unique collaboration with South Africa's township musicians set back the clock of South African liberation or drive it forward?