Gordon Brown and the Irish Taoiseach Brian Cowen are locked in talks in Stormont trying to prevent a collapse of devolution in Northern Ireland. The key issue is over the devolution of policing and justice powers from Westminster to Stormont, with Sinn Fein pushing for an immediate transfer and the DUP hoping to stave off the move until a later date. But why are these issues crucial to devolution? And why is a deal proving so difficult?