Complex sentence: The Liberal Democrats including their two predecessor parties amassed their largest share of the vote in 1992 and at the 2010 election their candidate was second-placed with 13.6% of the vote ahead of the Labour Party's candidate.
Split into simpler sentences: The Liberal Democrats amassed their largest share of the vote in 1992 (including results for their two predecessor parties). At the 2010 election their candidate was second-placed with 13.6% of the vote, ahead of the Labour Party's candidate.