When did the clock start clicking for Diana. . .

 Charles - Diana Wedding

Diana's marriage to Charles might have been a grand one, but certainly wasnt the happiest. Charles already had declared his love for Camilla Parker. In fact, newspapers reported that the night before his wedding, in July 1981, Charles and Camilla spent a night together on the Royal Train. Nevertheless, his love for Camilla kept on blossoming & they continued to spend time together.
         A very hesitant start of Diana's execution lies here. The "fairytale marriage" had broken down. Charles wanted to get rid of Diana so he could get along with her real woman. However, this wasnt a reason to convince anyone to get the Princess killed. Although all the events were unfolding in front of the British intelligence, those were considered pretty internal family matters. Meanwhile, his restlessness to break the marriage was on the rise. A chilling evidence of this will later be revealed after her death.
    For Diana's bad luck, the intelligence agencies wont be away from her for too long now. She was just on the brink of a journey which would break down all political & dynasty norms.