James Naughtie, known to one and all as Jim, is waiting for me at the bar of Edinburgh's Balmoral Hotel.

He has already ordered a glass of wine and he seems in his element among the tourists sampling their first malt of the weekend or enjoying a belated afternoon tea. He is 62, by and large bald, and carrying more pounds around his middle than is perhaps wise. Like reporters of old, he is wearing an indestructible tweed jacket, check shirt and trousers that will never need to be ironed. It is the uniform of a professional eavesdropper, of someone who is eager to merge with the crowd. Though he has lived for a large part of his life in London he has not acquired that sheen of metropolitanism which screams poseur.

For all his success, particularly as a presenter on Radio 4's Today programme, Naughtie has not changed in any fundamental sense. At first, he nestles my recorder in his groin, the better to pick up what he's saying, then elects to stick it in his shirt pocket. "It'll be all right there," he says, reassuringly, "and if it isn't you can just phone me up and we'll do the interview again."

His voice, one of the most recognisable in the land, is light and fruity and redolent of engagement. Above all, it's easy on the ear, like a purr. You do not need to be a linguist to place him. That he is Scottish is obvious; that he hails from Milltown of Rothiemay, near Huntly, is probably no less apparent. His accent, he says, hasn't changed that much. He was never immersed in the Doric though he has been known to slip into it when occasion demands. Rather, he is from that part of the world where it's regarded as an asset to speak English clearly and well. Class-wise, he could be a laird or a labourer.

The ostensible reason for our meeting is the publication of his first novel. The Madness Of July is a slow-burning, cerebral and gripping thriller which fuses the entwined professions of politics and espionage. It begins at Westminster, a milieu with which Naughtie is intimately familiar. From there it moves first to Washington, another of its author's stomping grounds, then to the Highlands and the estate where the novel's hero, Will Flemyng, was born and brought up and to which he retreats and watches the mist swirl over the loch "like the guilty secrets of a multitude of hidden smokers".

It's set in the seventies, when Naughtie caught his first glimpse of politics in the raw. That, he says, was the idea of his wife, Ellie Updale, a children's author and a former producer of The World At One. Significantly, it was a time when life was less technologically complicated. There was no internet, no home computers, no mobile phones. Pubs closed at 10pm and you walked on to a plane minutes before departure. If you wanted to contact someone incognito you stepped into a call box. Moreover, people still sent letters. Flemyng is an MP and a government minister but we do not know to which party he belongs, which may be taking the impartiality demanded by the BBC of its employees rather too literally.

Indeed, says Naughtie, amused at the thought, there is a lot about Flemyng he himself does not know. "I was asked recently, 'Where did he go to school?' Of course, I don't know. Maybe he'll tell me. I think it was certainly somewhere in Scotland, not England. I'm only beginning to get to know him. It's quite a good thing, I think. And I will get to know him better."

Flemyng is a sterling chap operating amidst shadows. In the oppressive heat of a London summer he must get to the bottom of a mystery which could destabilise the government and wreck his career. A body has been discovered in a cupboard in the Commons and on it is found Flemyng's phone number. But his creator seems less interested in cloak and dagger than in the interior lives of politicians whose careers hang constantly in the balance and whose domestic and emotional hinterlands are bywords for dysfunction.

In general, Naughtie says he's inclined to think the best of politicians, of whom he has encountered countless. "What I'm fascinated by are how close the light and the dark are in politics. How close ambition and great good are to disaster. And how you are always aware, if you are a sensitive and a bright person, that it's always provisional. That one day you can be in and one day you can be out … I'm emphatically a sceptic but equally emphatically I'm not a cynic. I think most people go into politics for broadly good reasons."

It was the late Arnold Kemp, then deputy editor of The Scotsman - later to become editor of The Herald - who persuaded Naughtie that his destiny lay at Westminster. Kemp, he says, was "a man I loved in many ways." At the time, 1977, Naughtie was working for the Press and Journal in Aberdeen. Kemp called him and took him to lunch and asked what he was going to do in London. Naughtie pointed out that as yet no job had been discussed let alone offered. Kemp laughed and that was that.

The first devolution "imbroglio" was in prospect, says Naughtie wistfully, and the Scottish National Party was on something of a roll. He recalls the political commentator Alan Watkins writing that "with the SNP every night was Burns night". "It was sort of true," he adds. The days were long and the nights short. Many bibulous hours were spent in the Strangers' Bar in the Commons, where you could routinely bump into big beasts such as Tam Dalyell, Enoch Powell, John Smith, Leon Britton and others. There were no spin doctors as such, and no pagers. "Those were electric occasions," says Naughtie, "they were diamond days, they really were. I saw politics at its most visceral in a way, a government hanging by a thread."

With acres of space in the paper to fill as Jim Callaghan's Labour government staggered onwards, Naughtie was like a fox in a henhouse. In essence, he performed a similar job to that of the young Charles Dickens, reporting verbatim from the fulcrum of power. It was an incomparable initiation, one which allowed him unprecedented access to men and women whose impact on Britain reverberates still. Even now, Naughtie cannot contain his freshman enthusiasm. What struck him was how altruism and ambition ran hand in hand with the imperative to do down enemies. For as he acknowledges, as politicians get closer to No 10, there will come a time - as in the case of Tony Blair and Gordon Brown - when friendship must be set aside in the pursuit of the ultimate prize. But where others might find this distasteful, Naughtie offers sympathy.

"One of my problems I would say - is it a problem? I don't know if it is a problem - is that I enjoy the company of real politicians, [those] who understand the contradictions of his or her trade. I never wanted to be a politician. But I have a kind of fascination for what they do and, in a sense, why they do it. I remember once talking to a Republican congressman who told me, 'The most painful thing is the sundering of friendship. The more successful you are the more friends you will lose.'"

In 1977, the Scottish national football team, under the management of the irrepressible Ally MacLeod, qualified for the World Cup finals in Argentina the following year. Immediately, Naughtie spotted an opportunity, telling Kemp he was keen to go there, embedding himself with Ally's Tartan Army. It was a "joyous adventure" which began even before his plane took off from Gatwick. "On board," he says, "were a party of South American nuns who had just finished what had been until that point a very successful pilgrimage." But to the nuns' chagrin, the other half of the plane was filled by the McEwan's Lager Fly To Argentina competition winners. Quite a time, recalls Naughtie, was had by all.

After stints at the Washington Post and The Guardian, Naughtie joined Today as a presenter 20 years ago and would like nothing better than to remain there until he's struck dumb. Whether he will, however, is a moot point. Three years ago he swapped Edinburgh for London as his main residence and for the last few months he has been dividing his time between Today and Good Morning Scotland, Radio Scotland's breakfast programme. The move was sold to him as an opportunity to convey to England the significance of what is happening north of the Border. As he acknowledges, there is a gulf in understanding between the constituent parts of the UK which he hopes to fill. But one senses that he is less than chuffed to be part of the GMS team.

"When the BBC talked to me about this last summer, it wasn't, you know, 'Go up to Scotland and join Good Morning Scotland', which of course I'm 'thrilled' to be doing," he says, raising his eyebrows. "It was do that, but also increasingly, over 2014, do stuff across Radio 4, making sure people who're listening in flood-wracked Surrey are understanding that there's a different view at two ends of Britain."

The perception in some quarters in Scotland was that he had been parachuted in to raise GMS's game. One former BBC presenter accused Naughtie of favouring the pro-Union camp while many on Twitter have decried him as out of touch. Naturally, he is not amused by such reactions. As he says, he has been journeying between London and Scotland for several years and, in any case, has always kept abreast of affairs here.

In London, meanwhile, the change in his circumstances has been interpreted as signalling the end of his sojourn at Today. It is perhaps a sign of its importance as an agenda setter that it engenders so much scrutiny, particularly in regard to the composition of its presenters and how they get on off air. One broadsheet journalist, Naughtie notes, in the course of an interview about his novel, said: "John Humphrys is a c***. You're not a c***. Why not? What do you think of that c***?" "Think of an answer to that question," adds Naughtie.

For the moment, he seems nonplussed, his legendary unflappability having deserted him. But it's not for long. He is used to being thrown brickbats as well as bouquets. Nothing much flusters him. When, for instance, Alex Salmond last week was caught in traffic and was late for an interview, Naughtie chuntered on regardless until the First Minister burst into the studio. Torture, one suspects, could not make him disclose which way he votes or which option he prefers in the referendum. Probably, it's more than his job's worth. But he does make for a frustrating interviewee when his fallback position is, "I'm not even going to go there."

The truth is perhaps mundane. He has been brainwashed by the BBC into having no opinion on such matters. Thus he is the conduit through which the public can grill their lieges, respectfully but not deferentially, the hallmark of the sceptic rather than the cynic. The same interviewer who failed to solicit his views of John Humphrys seemed to think him dour. But his disposition has always seemed to me to be sunny, the legacy, undoubtedly, of a childhood he regards as idyllic. "I had no angst at all," he says. "I had a really good time." Milltown, on the River Deveron, was surrounded by beautiful scenery, the like of which, according to one observer, was "seldom equalled, for the same extent, in any part of the kingdom." Both his parents were teachers and imbued him with a reverence for education. He roamed as he pleased and read widely. When he was 12, he wanted to go and see The Beatles in Elgin where they were due to play on Hogmanay. But bad weather meant the concert was postponed for three days. "I was too young anyway. My parents thought that the Two Red Shoes ballroom was a den of iniquity, something I later discovered to be absolutely true."

When he was 18 he went to the United States for a few months, spurred on by his love of Norman Mailer and other such writers. It was the beginning of an enduring affair. He is fond of quoting Alastair Cooke who said of America that "the fight goes on between its vitality and its decadence". Naughtie feels similarly about politics. He hopped aboard Greyhound buses and financed the travelling by working for six weeks in a kosher hotel in the Catskill mountains. There, he was employed as an assistant salad chef under the watchful eye of a Chinese cook from New York. Communication between the pair was tricky. "I couldn't understand him and he couldn't understand me." The confusion was compounded by Naughtie's inability to make a salad, most of the components of which had never been spotted in the environs of Huntly. "It was hilarious. I remember once laying out 400 portions of chopped chicken livers on the wrong coloured plates."

On his return home he went to Aberdeen University where he read English. His four years there coincided with visits from the likes of Iain Crichton Smith, Norman MacCaig, George Mackay Brown and Robert Garioch. Many were the "magical" evenings spent in such gilded company. His most inspirational lecturer was Isabel Murray - "one of those great people" - who taught him Jane Austen's Emma and much more besides. Ever since, he has been hooked on great writing and curious about what makes writers tick. On Radio 4's Bookclub he has interviewed many writers but until he wrote his own novel, he says, he had no idea what it entailed. What he has discovered is that until you write a book you don't have a clue what it involves. He's also discovered he's not Dostoevsky. But, then, who among us is? n

The Madness Of July by James Naughtie is published by Head of Zeus priced £12.99. The author appears at Aye Write! on April 10. Visit ayewrite.com.