It was a year of insurrection. In the streets of Paris and on the campuses of America, students were rioting. In Czechoslovakia the armies of the Warsaw Pact quashed the Prague Spring. Robert Kennedy and Martin Luther King were assassinated. North Vietnam launched the Tet Offensive. In 1968, amid worldwide chaos and disquiet, the BBC unveiled a quaintly nostalgic sitcom taking the mickey out of the Second World War.