Complex sentence: Following Labour's defeat at the general election in September 2014, he announced that he would resign as leader of the Labour Party and contest the subsequent leadership election but he then withdrew his nomination for the leadership election before the process started.
Split into simpler sentences: Following Labour's defeat at the general election in September 2014, he resigned as leader of the Labour Party and initially contested the subsequent leadership election. However he then withdrew his nomination for the leadership election before the process started.