In less than a month’s time, Queen Elizabeth II will mark 65 years on the throne, extending yet further the longest reign of any British monarch in history. However, her continued absence from public view has left many to contemplate a future beyond the only monarch most Britons have ever known.

Suddenly, the line of succession, which has remained a theoretical document since the Queen replaced her father, King George VI, in 1952, will be in the spotlight. It is a list, the length of which ensures that, no matter what fate should befall the royal family, there will be someone to continue a royal tradition in the country that stretches back more than 1,000 years.

A “heavy cold” forced Queen Elizabeth II  to miss the traditional Christmas Day church service for the first time in nearly 30 years. And she was again absent from the New Year’s Day service at Sandringham. Still, Buckingham Palace has sought to calm fears about the 90-year-old's health, with a spokesperson telling Britain's Independent newspaper that "she is continuing to recuperate and she is dealing with her regular red boxes of official papers."

In terms of what will happen in the event of the Queen’s eventual passing, strict plans are already in place. Prince Charles has long been the man waiting in the wings to take the throne as king. Indeed, whenever he does ascend the throne, he will be the oldest ever British monarch to start his reign.

At the age of 68, he is already significantly older than the previous record holder, William IV, who was 64 when he succeeded to the throne in 1830. He would last just seven years as ruler before his death meant the crown passing to his niece Victoria, as he was survived by no legitimate children.

There should be no such issues when the throne eventually passes on from Charles. Prince William is third in the line of succession, followed by his two children with Kate Middleton, Prince George and Princess Charlotte.

Royal family, prince charles, queen elizabeth, william, kate Prince Charles, Prince of Wales, Catherine, Duchess of Cambridge, Princess Charlotte, Prince George, Prince William, Duke of Cambridge, Prince Harry, Queen Elizabeth II and Prince Philip, Duke of Edinburgh stand on the balcony during the Trooping the Color, marking the Queen's 90th birthday at The Mall, in London, England, June 11, 2016. Photo: Ben A. Pruchnie/Getty Images

Any other royal currently on the line of succession is unlikely to ever ascend the throne.

William’s brother Prince Harry is fifth in line, but it would take a disastrous chain of events to those ahead of him in order for him to become king. The same is true of those below him, including Prince Charles’ brother, Prince Andrew, and his children.

But while many will only be aware of a few high-profile royals, the number of people on the line of succession extends into the thousands.

As a result of the Act of Settlement of 1701, devised to clear up disputes over succession, only the heirs of Sophia of Hanover can be rightful heirs to the throne. But, while the number of heirs was made finite, over three hundred years later it is a lengthy list.

Included among them is the ruler of Norway, King Harald V, who is around 75th in line to the throne and a second cousin to Queen Elizabeth II. Also on the list is Harald's nephew Haakon Lorentzen. The businessman, who grew up and still lives in Brazil, had been removed from the list after marrying a Roman Catholic but was reinstated following the passing of the Succession to the Crown Act of 2013.

Previously, marriage to a Roman Catholic made a person ineligible to ascend the throne, as the monarch is also the head of the Church of England.

That's not the end of the unlikely names on the line of succession. The thought of a man who has toured with Frank Zappa sitting on the throne may sound like the stuff of a Hollywood screwball comedy, but if a cruel fate were to befall a select group of 56 people then it could become a reality. James Lascelles is a long-time session musician who it so happens is also the first cousin, once removed, of Queen Elizabeth II.

For those looking for an even more romantic story of an everyday royal then look no further than Karin Vogel. A German therapist, Vogel was found to be the 4,973rd and last in line to the throne by two separate independent scholars. In 2011, she was tracked down by the Wall Street Journal and revealed her relief that she was unlikely to have to concern herself with reigning over the British Commonwealth.

“I can lean back and relax,” she said. “It is really very comforting that one doesn’t have to worry about Great Britain.”