NOW there are four people in this marriage. The Prince of Wales has invited two of his in-laws to join him and the Duchess of Cornwall on honeymoon in Scotland.

The newlyweds left Windsor Castle just after 6pm with the words “Prince” and “Duchess” daubed in soap or chalk on the windscreen of their car. The chav-style sign and the balloons tied to the rear had all the hallmarks of a Prince Harry prank. The car took them to RAF Northolt for the flight to Aberdeen.

From there the couple went to Birkhall, an unpretentious retreat on the Balmoral estate, where Annabel Elliott, younger sister of the duchess, and her husband Simon were to provide company for the newlyweds.

They were expected to join the royal couple for dinner on their wedding night, after an impromptu toast to the newlyweds by Birkhall staff on their arrival. One source said: “The two couples have been through a lot together,” and added: “The table will be laid for four.”

“The prince and Camilla are embarking on their second marriages and they are mature, middle-aged people,” said another. “It would probably be more unusual if they wanted to escape from everybody.”

It may be an unconventional start to married life but, according to those who know them well, it will provide significant pointers to the manner in which they approach the first year of their marriage.

Perhaps mindful of the mistakes in his first marriage, the prince is said by close associates to be anxious to treat his in-laws as equals.

“You should realise that the prince has gained a stepson and a stepdaughter (Tom and Laura Parker Bowles),” said the source. “He has always made his father-in-law (Major Bruce Shand) feel welcome by having him as a long-term house guest at Clarence House and Sandringham, as well as on past visits to Birkhall.”

The process of admitting Camilla Parker Bowles into full membership of the royal family has been arduous and she is determined to draw on the confidantes who stood by her during her “outsider” years.

Annabel, an interior designer, was close to her older sister during their East Sussex childhood, although their paths diverged during adolescence.

She married Simon, a property developer, and the sisters’ social lives became entwined again. It was at Annabel’s 40th birthday party that Diana, Princess of Wales confronted Camilla, accusing her of interfering in a marriage that she went on to describe as a “bit crowded”.

At the celebrations for Annabel’s 50th, held at the Ritz in 1999, Charles and Camilla appeared for the first time together for the cameras.

According to one source the duchess is reconciled to the need to take on a public role. The source said she had begun to display a new seriousness and devotion to duty in the weeks since the announcement of the wedding.

In keeping with other members of the royal family, she has surrendered control of her diary to her private office, starting with attendance at Crathie church today and the opening of a children’s playground in the nearby village of Ballater on Thursday.

Space has also been cleared for her to make a high-profile appearance alongside the prince at a charity gala in the West End of London in May.

In June, during Royal Ascot week, she and the prince will be host to a selection of wealthy American donors to Charles’s Washington-based foundation. The couple will host glittering dinners at Highgrove and Buckingham Palace.

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