I had to look up Marie Bonaparte's wikipedia biography to understand the Mitterrand anecdote. As it turns out, he did attend the coronation and, mysteries of protocol, was placed near princess Marie.

As the entry says: "On 2 June 1953, Marie and her husband represented their nephew, King Paul of Greece, at the coronation of Elizabeth II of the United Kingdom in London. Bored with the pageantry, Marie offered psychoanalysis to the gentleman seated next to her, who was the future French president François Mitterrand. Mitterrand obliged Marie, and the couple barely witnessed the pomp and ceremony, finding their own activity far more interesting."

If anybody has a picture of the princess attending the coronation, this is the time to come forward and post it. If the picture also contains young Mr. Mitterrand you earn a big lollipop in the shape of Queen Mary's coronation crown.