It must be daunting to find your family tree packed with heroes and villains, royals and landed gentry, military geniuses and a king-killer.

Is that what led Sir Ranulph Fiennes to lead a life of extreme adventure himself? I asked.

“No,” the explorer replied. “I didn’t know anything about my ancestors as a child growing up in South Africa. My mother told me tales of my father, who died in the war before I was born, and my grandfather who were both soldiers. I just wanted to follow in their footsteps.”

It was only later that Ran discovered he was descended from the Emperor Charlemagne, the first King of Jerusalem and a Magna Carta baron, that Broughton Castle has been home to Fienneses for 600 years, that Fiennes women bore eight kings, and, can you believe, the lady that rode a white horse to Banbury cross was a Fiennes.

His own extraordinary exploits as soldier, explorer and adventurer were described in his gripping autobiography Mad, Bad and Dangerous to Know (those words were his future father-in-law’s description of Ran to warn his daughter). It was only when Ran wrote a book about his family six years ago and then, this year, a book about the Battle of Agincourt in the run-up to its 600th anniversary, that he discovered so much more about his ancestors.

Like the fact that Fienneses were leading knights in the opposing armies of France and England. That in the Battle of Hastings ‘Cousin Eustace’ of Boulogne, William’s army commander, in the midst of battle advised the Duke they were about to lose and should retreat, but William didn’t listen.

“If only that bastard, William the Conqueror, had listened to my cousin Eustace’s advice and had retreated back to his ships, none of what followed need have happened” – the bloodshed of the Hundred Years War, the great Battle of Agincourt. “But William won the battle, Godwin collected an arrow in his eye and, sad day for the Anglo-Saxons, we Normans had arrived in Britain,” he grins.

Ran Fiennes kept his audience spellbound at the Henley Literary Festival in October at the launch of his book Agincourt. Always a lively and and entertaining communicator, he brought a unique perspective to that period of history, giving an irreverent poke – as the book does – at the characters who determined the fates of England and France.

Throughout the tale we meet his ‘cousins’ – sometimes noble, sometimes foolhardy, sometimes the right hand man of the monarch, sometimes traitorous, sometimes heroic. He loves referring to “my ancestor, the Emperor Charlemagne” or “my ancestor, the first King of Jerusalem”. He is equally happy to give grisly details of an ancestor who despatched King Edward II using a red hot poker where it couldn’t be seen, and of a lord-turned-poacher who was hanged.

Mad, bad and dangerous to know? – some of them, to be sure. Fienneses certainly made their mark on both British and French history.

Ran Fiennes made his own mark in the history books, after an early military career that saw him thrown out of the SAS for madcap pranks. His lifetime of adventuring has seen him become the first man to reach both poles by surface travel and the first to cross the Antarctic continent unsupported. He has led more than 30 expeditions, amputated his own frostbitten fingers, and aged 59 ran seven marathons on seven continents in seven days. He was named Best Sportsman in the 2007 ITV Great Briton Awards, and in 2009, aged 65, he became the oldest Briton to reach the summit of Everest. On the way, he has raised millions for charity.

So soldier, adventurer, record-breaker, historian, author of dozens of books... and Ran nearly followed in the footsteps of his actor cousins, Ralph and Joseph Fiennes. He says he was one of the final six hopefuls to replace George Lazenby as James Bond, back in the 60s, but the director declared Ran had “the face and the hands of a farmer. End of my movie career”. But his adventuring as “the world’s greatest living explorer” had barely begun.

* Agincourt, by Ranulph Fiennes (Hodder & Stoughton £20)