Stephen Stone was chief executive from 2002 to March 2018, when he became executive chairman. In April 2019 he became non-executive chairman.
From November, the new non-exec chairman will be Iain Ferguson, former chief executive of sugar producer Tate & Lyle plc. His non-executive board roles have included Balfour Beatty and Stobart Group.
Stephen Stone said: "It is a privilege to have served as chief executive and then chairman of this great company and to have worked with many exceptional people to create so much value for shareholders, employees and the communities where we operate."