Continuity mistake: Archer is at various times described as chief superintendent (a rank which wasn't introduced until 1949), superintendent and inspector. Nobody, including him, seems to know what his actual rank is.
Factual error: The constables working at Scotland Yard are wearing the divisional letter 'S'. 'S' Division covered North West London, including Hampstead, Hendon and Golders Green. Its men wouldn't have been anywhere near Scotland Yard.