Complex sentence: It was under no overall control until 1976, when the Conservatives took control until 1979, when it returned to no overall control; from 1980 to 2004 it was under Labour control.
Split into simpler sentences: It was under no overall control until 1976 when the Conservatives took control until 1979, when it returned to no overall control. From 1980 to 2004 it was under Labour control.