Complex sentence: He was the leader of Scottish Labour in the Scottish Parliament from 13 September 2008, to 17 December 2011, resigning after he oversaw his party's defeat in the 2011 Scottish Parliament elections.
Split into simpler sentences: He was the leader of Scottish Labour in the Scottish Parliament from 13 September 2008 to 17 December 2011. He resigned following his party's defeat in the 2011 Scottish Parliament elections.