If, for instance, they have heard something from the postman, they attribute it to ''a semi-official statement''; if they have fallen into conversation with a stranger at a bar, they can conscientiously describe him as ''a source that has hitherto proved unimpeachable.'' It is only when the journalist is reporting a whim of his own, and one to which he attaches minor importance, that he defines it as the opinion of ''well-informed circles.'' Evelyn Waugh