Riddell worked with the
International Bank for Reconstruction and Development in
Washington from 1969 to 1971. He was associate director of
First Boston Corp from 1972 to 1975, and director of First Boston (Europe) Ltd from 1975 to 1978. From 1978 to 1985 he was director of
CS First Boston, to which he returned as deputy chairman from 1990 to 1995. (deputy chairman from 1992). He was trustee of
The Frank Buttle Trust from 1981 to 1987, a member of the
Bloomsbury District Health Authority from 1982 to 1985, and deputy chairman of the
Independent Broadcasting Authority (IBA) from 1981 to 1985. He became an executive director of MC Securities Ltd in 1995. He was an Extra
Equerry from 1990 until his death. Riddell was appointed Commander of the Royal Victorian Order (CVO) in 1990 on his retirement as Private Secretary. He was made a Knight of the
Venerable Order of Saint John in 2002. He was appointed a
Deputy Lieutenant of Northumberland in 1990 and became Lord Lieutenant in 2000, stepping down in 2009. ==Personal life==