His gradual recovery, which included prolonged iron lung treatment, left him unable to attend school for four years.

Warren's final two films, Bloody New Year and Gunpowder (both 1987), were hampered by low budgets imposed by producer Maxine Julius.

His horror films developed a following, culminating in the making of Evil Heritage, a 1999 documentary about his work, and the release of a DVD box set in 2004.

In 2007, Warren worked on the supplementary features for the Region 1 DVD releases of Corridors of Blood (1958), The Haunted Strangler (1958) and First Man into Space (1959).

In 2016, Warren announced whilst being interviewed by journalist Steve Green[7] that he was in post-production on a new feature film, a thriller set in London's Chinatown.