He left SOE in 1946 and declined an offer of employment from the Secret Intelligence Service (SIS).
He went on to write marginally successful plays and films, including The Girl Who Couldn't Quite! (1947), Cloudburst (1951), The Best Damn Lie (1957), Guns at Batasi (co-writer) (1964), Sebastian (1968) and Twisted Nerve (1968).
Marks also wrote the script for Michael Powell's Peeping Tom (1960), the story of a serial killer who films his victims while stabbing them. Marks provided the voice of Satan in Martin Scorsese's The Last Temptation of Christ (1988).
In 1998, Marks published a book about his work in SOE — Between Silk and Cyanide: A Codemaker's Story 1941-1945. The book was reportedly written in the early 1980s, but didn't receive the UK Government's approval for publication until 1998. Three of the poems published in the book were scrambled into the song "Dead Agents" by John Cale and performed at a series of multimedia concerts in London in April 1999
He married the portrait painter Elena Gaussen in 1966, a marriage that lasted until shortly before his death at home, from cancer, in January 2001.
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