Complex sentence: On 12 September 2015 Corbyn was elected Leader of the Labour Party in a landslide victory, where he received 59.5% of first-preference votes thereby winning in the first round.
Split into simpler sentences: On 12 September 2015 Corbyn was elected Leader of the Labour Party in a landslide victory. Having received 59.5% of first-preference votes, he won in the first round.