"One can change one's citizenship, but not one's nationality. And I'm an Icelander through and through. It raises a few laughs at customs - they think it's unusual to see an Icelandic passport."
So does the king of quiz spend much time flying to and from Iceland? "I have visited Iceland six times a year for the last 68 years." (That's 408 times for those bereft of specialist knowledge in pure mathematics.) "We used to visit for family holidays when I was younger," he tells me. "But I suppose I really rediscovered my roots when I returned aged 17 to work on the herring boats in Reykjavik. In those days, the herring industry was all boom or bust. Luckily that summer we hit a real goldmine - I made enough money to fund my degree to read English Literature at Oxford. After I graduated, I returned to Iceland to lead tours, taking tourists around the sights of legend and sagas. We would stop at particular spots and the farmers would come out and tell us the ancient folk stories about the area.
"I've never travelled because I wanted to, though, but because I had to for my work. I never, ever go on holiday," he declares, almost proudly.
And which corner of the world might prompt a pass from Mr Magnusson?
"Iraq is the only country I really disliked," he says. "It's such a difficult place, the people don't want you to do any filming there, the hotels are vile - it's really the arsehole of the world ..."
Er-hem ... and the upper brainstem of the world?
"It must be Crete. We went to do some filming there in 1974. There's something about the island's uplands that reminds me of Iceland. The men were wonderfully manly, too - at least, in the days when it was fashionable to say things like that. At one point, on my way back to the airport, I stopped in a cafe full of Greek locals. And there, in the middle, was Nobel prize- winning poet, George Seferis, reciting poetry. I'd done a little classical Greek at school, so I could understand a few words. It was a truly wonderful experience."
'The Natural World: Fire and Ice - An Icelandic Saga' will be narrated by Magnus Magnusson on BBC2, Sunday 1 March at 6.40pm.