Complex sentence: The security service (MI5) carried out surveillance of CND members it considered to be subversive and from the late 1960s until the mid- 1970s it designated CND as subversive by virtue of its being "communist-controlled".
Split into simpler sentences: The security service (MI5) carried out surveillance of CND members it considered to be subversive. From the late 1960s until the mid- 1970s, MI5 designated CND as subversive by virtue of its being "communist-controlled".