Complex sentence: Although no longer serving as Prime Minister, Cameron originally stated that he would continue in Parliament as a Conservative backbencher; however on 12 September, Cameron announced that he was resigning his seat with immediate effect.
Split into simpler sentences: Although no longer serving as Prime Minister, Cameron originally stated that he would continue inside Parliament, on the Conservative backbenches. On 12 September however, Cameron announced that he was resigning his seat with immediate effect.