Cadell was a member of the National Youth Theatre and appeared with them in the 1967 production of "Zigger Zagger". He trained at the Bristol Old Vic Theatre School. His first successes were found in the theatre in the mid to late 1970s. An early television role was in Simon Gray's 'Play for Today's in 1975 "Plaintiffs and Defendants" and the sequel "Two Sundays" both opposite Alan Bates. He then provided the voice of Blackberry in the animated adaptation of "Watership Down" (1978), based on the novel by Richard Adams. Subsequently, he had roles in television programmes such as "Enemy at the Door" (1978–80), and also appeared briefly in the disaster film "Meteor" (1979) as a TV news reporter.