Jason started his television career in 1964 playing the part of Bert Bradshaw in "Crossroads". In 1967, he played spoof super-hero Captain Fantastic, among other roles, in the children's comedy series "Do Not Adjust Your Set" (Rediffusion London/ITV) with Eric Idle, Terry Jones, Denise Coffey, and Michael Palin. Humphrey Barclay, who recruited Jason to appear in "Do Not Adjust Your Set" (partly to counter the more highbrow style of Idle, Jones, and Palin), admired his sense of timing. The programme ended in 1969, and the character then appeared for a time in the Thames Television children's programme "Magpie".