Complex sentence: Lucker left the series halfway through series four, though she returned midway through the next series, by which time the series' ratings had fallen significantly, therefore certifying Lucker as the main star of the show although the series was cancelled in 2006.
Split into simpler sentences: Lucker left the series halfway through series four, though she returned midway through the next series. However, by this time the series ratings had fallen significantly, therefore certifying Lucker as the main star of the show even though the series was cancelled in 2006.