Complex sentence: From 2011, Jeremy Corbyn was the chair of the Stop the War Coalition, but a week after his election as leader of the Labour Party in September 2015 announced that he was stepping down from the role, but would continue to support the group.
Split into simpler sentences: From 2011, Jeremy Corbyn was the chair of the Stop the War Coalition. A week after his election as Labour leader in September 2015 announced that he was stepping down from the role, but would continue to support the group.