by ANDREW SINCLAIR
328 pages. Harper & Row. $6.95.
Artifactually, Magog is one of two wooden statues in London's Guildhall. The other statue is Magog's brother Gog. Symbolically, they are London's janitors. Mythically, they are the survivors of a race of defeated giants. An odd couple whose meaning is obscured by the mists of prehistory, they suggest the dual nature of a single being. Brought up to date by the English novelist, playwright and historian, Andrew Sinclair, Gog and Magog come to signify the haunting memory and failing desire of a geratic Britain....