Simon Tranter was a character who died 30 years prior to the events of the episode Death of a Stranger.
He was the son of the former owner of the Tranter Estate and the husband of Marcia Tranter. Simon Tranter was sterile and knew that, so when his wife got pregnant with a child from her affair with Michael Darrow, he knew that it couldn't be his. Marcia got James Fitzroy to kill Simon and bury his body in the wood. Then they sent Darrow away and paid him to use Simon's passport to make it appear that Simon had run off with another woman.
With Simon out of the way, Marcia was free to cast her son Grahame as Simon's heir. After seven years, Simon was declared legally dead, so as his Simon died without leaving a will, Grahame inherited the Tranter estate as the nearest next of kin.
Years after his death, Simon's remains were discovered by an employee of Fitzroy, who was traumatized by the discovery and was sent to a home for the elderly paid by Fitzroy to keep him from talking. But when Michael Darrow (the Tramp) and Ben Gurdie were murdered, Barnaby organized a search in the forest where Simon's collarbone was found.
After James Fitzroy learned that he wasn't Grahame's father, he confessed to having killed Simon, saying that Marcia made him do it.