''I thank heaven for small mercies. The first of these is Rumpole'' Clive JamesHorace Rumpole, the irrepressible barrister fuelled by cigars, Tennyson, steak-and-kidney pud and the cooking claret from Pommeroy''s wine bar, is back for further misadventures. Amid an unfortunate and temporary downturn in London crime, the Old Bailey Hack sits in Chambers (he never writes at home for fear of She Who Must Be Obeyed) and picks up his pen to recount six classic tales of his recent trials. Here he deals with, among others, a clergyman on a shoplifting rampage, a backstage theatrical murder, a villain with unfortunate sartorial taste and, worst of all, the possibility that he may have to hang up his wig and retire. ''Rumpole, like Jeeves and Sherlock Holmes, is immortal'' P. D. James