 In 1956, Imbert joined Special Branch, learning shorthand and Russian during his 17 years with the unit.In 1973, he was made deputy head of the Anti-Terrorist Branch, where he became an expert on European terrorist groups such as Baader-Meinhof, and gave lectures on hostage negotiation and counter-terrorism tactics.On 6 December 1975, four members of the Provisional IRA barricaded themselves in a flat in Balcombe Street, Marylebone with two hostages.