GUARDIAN ANGELS: A giant rock group tour is being planned, with top names from the United States, and security is the big problem with the promoters. How can they avoid the appalling violence from rioting fans, without jeopardising the lives of the security guards themselves? The Hell's Angels seem the answer, and Gerry Vinson's Last Heroes emerge from their Welsh retreat to do the honours. But the American groups have organised their own protection - an American chapter. The inevitable rivalry