Chegwin's most prestigious acting role was that of Fleance in Roman Polanski's film Macbeth (1971).
This rivalry between groups of fathers and sons (Banquo and Fleance, Macduff and his son, Macbeth and his lack of a son) is seen as an important theme of the play.
Even if he had not died in 1055, Siward would be highly unlikely to have survived the aftereffects (Norman replacement of Saxon nobility) of Hastings; in Lukeman's play, Siward is still alive—and is one of two (the other being Seyton) who repeatedly advise Malcolm to move against Donalbain and Fleance