The Four Just Men

The Four Just Men was another ITC production of crime and intrigue. The "four" were, or the actors that played them were, Richard Conte, Dan Dailey, Jack Hawkins and Vittorio de Sica. All four only appear in the first episode together and the following episodes featured only 1 on a rotating basis. It was derived from the Edgar Wallace novel of the same name.
http://en.wikipedia.org/wiki/The_Four_Just_Men_%28TV_series%29

Though over 50 years old familiar faces from more recent times do appear.

Honor Blackman is in the Dan Dailey episodes as his secretary/lover.

Frank Thornton, now approaching his 90th birthday,  appears in 9 episodes in nine different minor roles.

And a 25 year old Judi Dench appears in Treviso Dam.