David Cameron stunned Tory party by ruling out seeking a third term . Insisted he will serve 'every day ' up to 2020 if he is re-elected on May 7 . But Iain Duncan Smith says he will have to stand down earlier . PM tipped Boris Johnson , George Osborne and The Fixed-Term Parliaments Act May as leaders .
David Cameron stunned Tory party by ruling out seeking a third term . Insisted he will serve 'every day ' up to 2020 if he is re-elected on May 7 . But Iain Duncan Smith says he will have to stand down earlier . PM tipped Boris Johnson , George Osborne and Theresa May as leaders .