He marries Veronica Duncan in March 1963, and they move to Lower Belgrave Street, in affluent Mayfair, a short distance from the Clermont Club. John inherits the Lucan title, becoming the seventh Earl, when his father dies two months later. Veronica, Countess of Lucan as she was now known, gives birth to three children between 1964 and 1970, but suffers severe postnatal depression after each delivery, which is treated with a variety of anti-depressants that compromise her mental well-being over subsequent years (although she continues to care for her children.)
Although initially sympathetic to his wife’s condition, Lucan becomes increasingly dissatisfied with his wife’s behaviour and the marriage deteriorates. Mounting gambling debts also add to the pressure and there are reports that he has become violent towards his wife. The marriage disintegrates in 1973, and Lucan moves out of the family home in Lower Belgrave Street and into a garden flat, in Elizabeth Street, nearby.
He tries unsuccessfully to get custody of the children, by hiring private investigators to spy on his wife and goads her into violent exchanges on the telephone but, despite his best efforts, she is granted full custody in June 1973. By this stage his debts have mounted considerably, and his acrimony towards his wife is increased further by the fact that she resides in the family home that, if sold, would solve his financial problems immediately. He is reported to have told more than one person that he wanted to kill his wife, to get back his home and his children.
Fearing for the safety of the children, the police break into the house and discover them safely in bed. But they do find the body of Sandra Rivett in a bag in the basement, as well as a length of lead pipe, wrapped in tape, near the front door. Lord Lucan has disappeared, and a search of his nearby flat reveals only that his wallet, passport, driving licence and car keys were all still there.
Lucan had, in the meantime, tries unsuccessfully to summon the help of a nearby friend, Madeleine Floorman, and then calls his mother to tell her that Lady Lucan had been injured and instructs her to collect the children from the house. She arrives to find the police already there, and she takes the children home with her.
In the meantime Lucan takes another car, which he borrows from a friend, and drives down to the Surrey home of Susan Maxwell-Scott, a close friend, where he tells his own version of the story, which was in stark contrast to his wife’s view of events.
He says that he had been passing the house that night when, looking inside, he had noticed his wife struggling with an unknown man, at which point he went to her assistance, letting himself in and going down into the basement, where he slipped in a pool of blood. The man then ran off, and his wife became hysterical, accusing him of hiring someone to murder her. When he tried to help her, she ran away and he realised that it would be best if he left the house.
He reinforces this version with a letter to his brother-in-law, Bill Shand-Kydd, whom he had been unable to reach on the phone, in which he emphasises his wife’s mental condition, suggesting that she is suffering from paranoid delusions. He calls his mother again, and she advises that the police are at her home, asking to speak to him. He promises to be in touch the next day.
Recognizing the gravity of his position, Lucan leaves Maxwell-Scott’s home at 1.15 am in the borrowed car, and is never seen again. The car is recovered in Newhaven some days later, and the police find a lead pipe similar to the murder weapon inside. The owner of the vehicle receives a note from Lucan in the post, protesting his innocence and putting events down to unfortunate coincidences, stating that his main concern was now to protect the welfare of his children.