Imagine the sound of Approaching Menace music, the inquisitorial spotlight, the camera zooming in, the terrified man sitting in the famous black leather chair, the nervous flinching and twitch of the mouth . . . this afternoon we have polymath Magnus Magnusson in the hot seat with 45 minutes on The Life and Reign of Magnus Magnusson, Mastermind quizmaster.

This is the beginning of the first, nail-biting round. ‘How about a bottle of medium white wine?’ asks Magnus, showing his virtues as questionmaster. ‘Oh, do we have to?’ squeaks his producer. They then do battle for the title (of ‘dry’ or ‘medium’). ‘Medium dry!’ says Magnus, showing a talent for compromise. Beep beep beep. That brings us to the end of this fast, furious and high-scoring round.

Magnus, you’re a writer, broadcaster, scholar and chairman of the Scottish National Heritage Board. You have an honorary knighthood for your service to National Heritage, but as an Icelander you can’t be called ‘Sir’. You’re not allowed questions on orthopaedic bone cement in hip replacements or routes to anywhere in mainland Britain from Letchworth by road, but you can be quizzed on anything from notable British poisoners to the life cycle of the honey bee. Your chosen specialist subject? ‘Icelandic sagas or the history of Lindisfarne.’

Right. You’re 63. Are you going to retire as Mastermind quizmaster? ‘Mastermind has lasted 21 years. I see no reason why it shouldn’t last until 25.’ He clears his throat defensively. ‘The game depends on speed. If I did get slower, I think I’d like to be put mercifully out to pasture.’ Correct. How long have you been married to Mamie? ‘Thirty-eight years, 10 months.’ That’s astonishing and unusual, isn’t it? ‘It’s not unusual,’ he says, frostily. ‘More people remain married than don’t get married.’ What’s the secret of marital longevity? ‘Marrying someone nice.’ What else? Pass. What has he learned from his marriage? ‘That it would be much too tedious to separate.’ What else? Pass.

He’s a kind, soft man with gravitas and dignity. But he’s initially prickly and shirty if a question is any more intrusive than ‘What’s the monetary unit of Japan?’. He’s also pedagogic.

He sports a blue/green jacket, green tie with an otter on it (‘from the island of Isla which I visit quite a lot because we have a goose problem . . .’) and blue eyes. His hair is silver and he trims his eyebrows. ‘It’s genetic. My father needed a topiarist to deal with his.’ He sticks a pencil in his pipe, rakes the tobacco, lights it, never smokes it and looks like a pre-Socratic philosopher.

Back to the quiz. Tell us some funny stories about Mastermind, about a contestant passing on all the questions, for instance. ‘That’s not funny. That’s tragic,’ he says, stormy-faced with frightening intensity. ‘Our job is to bring out the best in people. There was one woman who sang her last answer.’ Correct.

You missed out the laboratory technician who was disqualified because she entered twice, first as Mrs Denyer, her first husband’s name. ‘Oh yes,’ says Magnus, nodding. ‘She didn’t think she was doing anything wrong. She felt she was a different person having had a bad marriage, remarried, changed her hair and reconstructed herself.’ Correct.

Instead of the crisp tones and fast delivery seen on television, he speaks in a soothing broadcasterly Scottish burr. He’s also the sort to pronounce correctly any Japanese, Swahili or Gujarati words. Why is the programme so popular? ‘We’re trying to celebrate what the human noddle can do . . . and at home you compete, shout answers at the screen and think, I got one quicker than those brilliant little bastards! I always think that when I watch University Challenge and see those hideously, formidably brilliant little sods!’ Correct.

How would you define Mastermind contestants? ‘They’re people who like taking part in quizzes. I also think they’re crazy. You’ve got to be a wee bit potty to go in for Mastermind. I wouldn’t for all the tea in China.’ Why? ‘You’re putting your pride on the block. By applying, you’re implying you have a certain good conceit of yourself and there’s a danger that you’ll come a cropper.’ Correct.

Who is the questionmaster’s questionmaster? ‘My ideal quizmaster is Bamber Gascoigne, the king, the best ever,’ he answers with conviction. ‘If I were as good as him, I’d reckon I’d done a pretty good job with Mastermind.’ Are you not as good as him? ‘Of course not. Nobody is. He’s miles above everyone.’ Do you know your subjects? ‘On the night,’ he laughs. ‘Then I know all the possible answers to all the questions.’ Correct. If you were given six months to live, what would you do? ‘I’d carry on doing what I’m doing now. I hope I wouldn’t be thrown into a flat spin by it. I think I would cultivate my garden a bit more.’ Do you want that quote in French? ‘Ah, you know your Voltaire.’

How would you describe yourself? ‘As an ordinary guy who got notoriety through doing parlour games on television.’ Beep beep beep. Let us glance at the scores. Magnus, you’ve scored 50 points and several marital passes. It’s easy to play games. But Magnus has had tragedy in his life. Siggie, his 11-year-old son, was killed in 1973 as he jumped off a bus on his way home from school. ‘It’s curious,’ he says, softly, ‘but in a perverse way his death brought us all closer together and made us even stronger as a family. We’d taken for granted that we were all immortal and happy, and that things only happened to other people. You realise then you can’t take happiness for granted. That you have to work at it and help create it. His loss was a grievous and shattering blow.

‘I remember him a bit like Keats’s Grecian Urn. He was just on the threshold of puberty. He’d started worrying about whether he should kiss a girl in his class. The night before he died, he came and sat on my knee and said, ‘Dad, do you think it’s OK if I kiss her?’ I said, ‘Of course’, and he ran off to play.

‘We often wonder what path he would have taken. He’d have been 31 now. He’d probably have gone into broadcasting.’ His four children are in broadcasting. Magnus smiles and shrugs his shoulders. One is left with the impression of a gentle gentleman with great intellectual curiosity. * The Mastermind final is on 16 May.