Who killed Rex?

Lancelot Fortescue masterminds the murder of his father, Rex, his stepmother, Jennifer, and the family maid, Gladys Martin. Lance does it for the inheritance he will receive from his father, including the Blackbird Mine in Africa. He needs to get his stepmother out of the way because if she dies within thirty days of her husband, her large inheritance will revert back to the estate.
Lancelot has posed as Albert Evans and wooed the maid Gladys. He persuades her to administer the poison to Rex in his marmalade, telling her it is a truth serum. He then strangles her to silence her. In a macabre retelling of the nursery rhyme "Sing a Song of Sixpence," Rex, inspired by the name Blackbird, pins a clothespin to Gladys's nose to mimic the actions of the blackbird in that rhyme and arranges for a pocket of rye to appear in his dead father's pocket.
This is a Miss Marple mystery, and through a coincidence, Gladys happens to have once been Miss Marple's maid. When Miss Marple returns home, having solved the case, she finds a letter from Gladys describing her part in the plot.

Rex Fortescue was murdered by his son, Lance, operating under the guise of Albert Evans. He instructed his devoted girlfriend, the simple-minded parlor maid Gladys Martin, to put poison in Rex's marmalade. As part of his fiendish plot, Lance was scheduled to be out of the country when his father died. Getting the dim and hopelessly smitten Gladys to do his dirty work would provide him with a perfect alibi. No one would suspect him of his father's murder.
Lance is already responsible for murdering his stepmother. He got rid of her because her prospective inheritance would have leeched much-needed cash from Rex's business. Lance will also go on to kill Gladys in order to keep her quiet. His motive for murdering Rex is simple: money. Rex owned the deeds to the Blackbird mine in East Africa. Now that uranium has been discovered there, it could become a very valuable property indeed, and Lance wants to get his greedy hands on it.