The monarchy is the ultimate exclusive club, the sovereign born to reign over subjects, not citizens, hardly the most obvious of qualifications for performing the key role of a head of state: to unify. The Queen has nevertheless largely succeeded in this function, popular and, in decade after decade and poll after poll, trusted, rarely faltering but for a few famous missteps, her delays in leading public mourning for the dead after a mining disaster in Aberfan, Wales and, later, for her daughter-in-law Diana.