But his files are sealed, because someone else whose name appears in the same documents has asked that they be removed, leaving a mystery about the events surrounding his visit and his arrest.
Along with several others, he came down to Jackson in 1963 from Yale, where he had been an exchange student. "As a firm supporter of the United States, I felt I had a stake in the issue," he says. He worked with the Rev Edwin King, a civil rights worker, and Aaron Henry, who were running a "shadow election" to show up the bankruptcy of the segregationist white rulers of the state. But while he was there, the conductor Sir Malcolm Sargeant visited Jackson with the Royal Philharmonic Orchestra. They were due to play for a whites-only audience, and Mr Bosanquet tried to dissuade him. Sir Malcolm would not listen, and went ahead. On his way to the concert, Mr Bosanquet and two others were arrested. He was freed three days later, and afterwards the musicians' union modified its rules on playing in segregationist states.
At the time, the level of violence was relatively low; later, things got much more dangerous. The next year, three students were murdered. "He was risking his life," said Mr King, a civil rights worker. The Englishman returned, and worked with the civil rights movement again. "I'm very proud of what was achieved there in the face of massive opposition," said Mr Bosanquet. "We were young, and even more idealistic than now."