The 5th Earl of Balfour first realised the significance of the Balfour Declaration when he was a 14-year-old schoolboy at Eton, hailing a taxi while on holiday in London.
A copy of the Declaration - made a century ago by Arthur Balfour, the former prime minister who later became foreign secretary, backing the creation of a Jewish state in Palestine - hung on the wall of his loo at home, where the young Balfour read it without really appreciating its importance. The iconic document was never discussed at home or school.
So it was a crucial moment in Lord Balfour's life when a Jewish London cabbie spotted his school trunk with his surname painted on it.
"Is this yours, mate? Are you anything to do with the prime minister?" (His ancestor, the 1st Earl of Balfour, served as Conservative Prime Minister from 1902 to 1905.)
"Yes," he said, explaining he was his great-great nephew.
"I don't believe this!" said the taxi driver. "Wait till I get home and tell my family. What he did for us! Tonight, it's Passover and you're my last fare before I knock off to go off to the East End for Passover dinner." The driver then pulled off the road and started singing Jewish songs to the teenage boy.
"I was so bowled over by this that I was late, and I dashed out of the cab, leaving a family picture in the back of the cab," says Lord Balfour, 68, a banker. "I thought it was now in the East End. Instead, he went to the lost property office, all the way up in north London, deposited it, stuck a label saying Balfour on it. I was able to find it the next day.
"That was my first time I realised the importance of the declaration to Jewish people."
Today, a fine Philip de László portrait of Arthur Balfour presides over Lord Balfour's drawing room in his handsome Sussex house, where he lives with his wife, Lady Tessa, a daughter of the Duke of Norfolk. They have four daughters, all interested in their ancestor's 100-year-old Declaration.
The Declaration was sent by Arthur Balfour to Lord Rothschild, the de facto secular head of British Jewry, on November 2, 1917. It was then passed on to the Zionist Federation of Great Britain and Ireland.
The short, single-page document declared: "His Majesty's government view with favour the establishment in Palestine of a national home for the Jewish people, and will use their best endeavours to facilitate the achievement of this object, it being clearly understood that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine, or the rights and political status enjoyed by Jews in any other country."
The Declaration was absolutely vital in the long build-up to the eventual creation of the state of Israel in 1948 - which also happens to be the year the current Lord Balfour was born.
Despite that teenage meeting with the taxi driver, the Balfour Declaration played little part in young Roderick Balfour's life. "It was never mentioned by anybody at school, or very much as I grew up," he says. "At school, everyone said: 'Are you related to that loser Balfour?', because he lost the 1906 election. They didn't know about the Declaration.
"But you go to Canada, Argentina, France or anywhere in the [Jewish] diaspora, and they all know about it. This country has less knowledge than anywhere else."
In fact, Lord and Lady Balfour have just returned from a seminar - "From Balfour to Brexit" - in Jerusalem. Whenever he goes to Israel, he is reminded of the affection in which his great-great uncle is held.
"People come running up to you and just say: 'Thank you,'" he says. "I started going to Israel on bank business in the 90s and saw Balfour Streets in every town. The Prime Minister lives today on Balfour Street."
However, Lord Balfour sees imperfections in the modern state of Israel. "I have major reservations," he says. "There is this sentence in the declaration, 'Nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine.' That's pretty clear. Well, that's not being adhered to. That has somehow got to be rectified. Talking to the more liberal elements of Jewry, they would acknowledge there has to be a greater economic role for the Palestinians."
It was only in the 1990s that Lord Balfour really began to appreciate the magnitude of his ancestor's Declaration. In 1990, he started working at NM Rothschild, the merchant bank run by Sir Evelyn de Rothschild, a cousin of the Lord Rothschild to whom Arthur Balfour addressed his Declaration.
When Lord Balfour joined the bank, Sir Evelyn de Rothschild asked him: "What sort of a Balfour are you? Are you Balfour Beatty [the construction company]?" ("I said I'm one of the poor, government Balfours," Lord Balfour replied.)
And then, in 1992, on the 75th anniversary of the Declaration, Lord Balfour saw in the papers the list of attendees at the anniversary dinner; there wasn't a Balfour among them. He got in touch with the Anglo-Israel Association and asked if he could get involved with future events.
This year, he and Jacob Rothschild - the current Lord Rothschild, whose forebear Walter Rothschild received the Declaration - will host a dinner on the anniversary in a government venue in London. "There are a huge number of events going on on the same day," says Lord Balfour. "Jacob Rothschild is very kindly organising a dinner, which he and I are nominally joint hosts of, although it's very much his initiative."
Lord Rothschild's team have located descendants of those involved in the Declaration from around the world. Among them will be a Lloyd George, in honour of David Lloyd George, prime minister at the time of the Declaration; and a member of the Sykes family: Sir Mark Sykes, Bt, devised the Sykes-Picot Agreement of 1916, which laid out national boundaries in the Middle East.
Next month, there will also be an event at the Royal Albert Hall to commemorate the Declaration, which Lord Balfour will attend with his family. The event will include a 500-voice massed Christian choir, a Klezmer band, the Israeli Dance Institute and the Israeli singer Tally Koren.
Lord Balfour never met his famous ancestor - he died in 1930, aged 81. But he speaks of him with great affection. "We all knew about Arthur James because he had been prime minister, and the family were immensely proud of him," he said. "We all knew him as Nunkie, although I never met him. My father was nine when he died, so he knew him well. He was very much loved."
Arthur Balfour was an intellectual - "primarily a Bible-reading philosopher", says Lord Balfour - but a well-connected one, too. He was a leading member of the Souls, an elite salon of Victorian upper-class intellectuals. Balfour's uncle, the Marquess of Salisbury, who was known to the family as Bob, had been prime minister before him: "That's where the expression 'Bob's your uncle' comes from. In other words, it's quite easy to get on if 'Bob's your uncle'."
But rather than a popular turn of phrase, it is for his Declaration that his family would like him to be remembered.
"It was a great humanitarian gesture," says Lord Balfour. "Humanity should be extremely grateful."
• Harry Mount is editor of The Oldie Magazine