The fact Benton is a warrant officer, having been promoted in the first Fourth Doctor serial, Robot, places it after that serial, but presumably prior to his departure from UNIT in the late 1970s as revealed in the serial Mawdryn Undead. In September 2015, the film was released on DVD On his way to UNIT HQ in order to deliver some radioactive materials, Warrant Officer John Benton has an unexpected flashback to his childhood in Lancashire. After visiting the grave of his older brother Chris, Benton has a further flashback to his childhood in 1944.
Chris
Benton