Relph is largely known as a film producer. He served as associate producer on the Ealing comedy "Kind Hearts and Coronets" (1949); and had a significant 20-year partnership with Basil Dearden beginning in 1949 and ending with Dearden's death in 1971. Their work included a series of social problem films examining issues such as racism ("Pool of London" and "Sapphire"), juvenile delinquency ("Violent Playground"), homosexuality ("Victim"), and religious intolerance ("Life for Ruth"). Relph believed that because film was "genuinely a mass medium," it therefore had "social and educative responsibilities as well as artistic ones." In their review of "Life For Ruth", "The New York Times" wrote, "in avoiding blatant bias, mawkish sentimentality and theatrical flamboyance, it makes a statement that is dramatic, powerful and provocative."