Jane today in the study at Saltwood
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Alan Clark lies at Saltwood Castle under the lawn he mowed. A 6ft rock of fossil-encrusted, honey-coloured sandstone marks the place where he is buried alongside two of his beloved dogs, enclosed by an arc of stone benches and newly-planted rosemary, for remembrance.

"I'm so glad he's not in the churchyard. I'd hate people to notice every time I visited the grave: 'Mrs Clark's been here again. Mrs Clark's not been here for a long time.' It wouldn't be the same. It's much more comforting to have him here where I can sit and talk to him."

What about? "Everything," Jane says. Saltwood is, as Alan said, "one of the loveliest places in the whole world".

Jane drives me past grazing sheep, snoozing peacocks and scuttling moorhens, the moat, through the gatehouse with signs warning that rottweilers prowl here. Into the donnish study, where I once sat over sherry by a crackling fire while Alan drawled on about arms to Iraq. On this chilly overcast afternoon, the room is fireless, and we talk of him.

Jane's hair has grown long, with threads of grey, but her face and laughter are girlish as ever and she wears her usual teenagerish jeans. She had just driven down from their vast Highland estate, Eriboll, 716 miles in a day."Till death us do part," she adds, "means the death of both of you. The first time I did that drive alone, after he died, I found stopping at motorway cafés incredibly hard. Really upsettingly difficult. I'd never gone into those places on my own. How do you sit alone at a table if you're a woman?" Take a book, I said. But she likes observing people. So this time "it was 'Drive on, driver' - that's what Al used to say," and on she drove.

Jane today in the grounds of Saltwood with one of her dogs
Photograph: MITCH JENKINS

Arriving back after midnight, she went into the moonlit garden and tipped a bottle of Eriboll seawater over his stone.

In Scotland she sat in her croft overlooking the sea. "I'd taken masses of stamps to reply to people who wrote on the anniversary of Al's death. But I just gazed endlessly at the sea. It's so peaceful there it recharges your batteries. I collected wood and peat, and bathed in the icy sea, as Al would have done. I took off my Barbour and layers of jersey and hurled myself in. The sea is so clean. And the light is beautiful, even in horrid weather."

She is a talented painter, but hadn't taken up her watercolours since her last Christmas card, white snowdrops on black background. In Scotland she painted just three things: a pea-pod, a seashell and a dead fieldmouse the cat brought in.

Jane Beuttler was only 14 when Alan, in his late twenties, picked her up on Camber Sands, the beach at Rye where her family and the Clarks both lived then. They married two years later and her dress had to be narrowly fitted to her 22-in waist, Alan insisted, because everybody assumed she must be pregnant.

Their honeymoon was spent on the Somme - Alan was still writing The Donkeys, his indictment of the First World War generals - and in Zermatt and Positano, where a girlfriend of his turned up. "She was very nice, I liked her," Jane says. (In Michael Cockerell's film about the Clarks, Love Tory, she said: "We could have had a Buńuel situation and done away with him.")

Later she often seethed about Alan's women: "Bluebottles," she called them, "some harder to swat than others." He was impossible, but she knew he adored her. "Al's a monster, but he loves little Jane," as her mother-in-law said.

They were so close they finished each other's sentences. And he depended on her, domestically helpless: when she was with her mother in Spain, he would ring thrice a day, asking how long to boil potatoes. She would say to her mother: "Tell him I've run off with a dashing Spanish count."

She did walk out, once, to teach him a lesson. "It was so boring, I can't tell you." She drove herself to Hordle, her birthplace in the New Forest, to see the grave of her sister who died in infancy. "But on day two I remembered I hadn't watered the greenhouses. There was nothing for it but to come back."

Alan knew she'd be back: he set her place at the table. "I had no intention of leaving," she says. "Anyway, if you know you're loved, it's fine. We were soulmates. And he was enormous fun. He just loved attention. Most men love attention, don't they?"

Not to his extreme, no. But that was part of the testosterone-driven charm. Journalists loved him because he was recklessly candid and wickedly quotable. He was dangerous, he made politics more exciting and you didn't have to be a Tory to appreciate him. When David Frost asked, in his 70th year, if he had given up wine, women and song, he replied languidly: "Well I've never been particularly addicted to song."

The new volume of diaries is a prequel to the first, covering the years 1972-1984, when he became Tory MP for Plymouth Sutton. Being selected was, he writes, the best day of his life since the birth of James in 1960: a long labour, a 9lb 5oz baby.

The young Jane Beuttler on the day of her marriage to Alan Clark
Photograph: PA

They came to live at Saltwood then, on the top floor with the inlaws below. "It was February, and the lambs made the same noise as the baby, so we were never sure who it was."

Alan was working on this volume before he died. Jane was at first reluctant to look back on those years of Cabinet ambitions, debts, girl troubles including a spot of blackmail, but Michael Sissons, the literary agent, persuaded her that a writer belongs to his readership, and she must share her incendiary husband.

She never realised how anxious he had been about money. "He would say, 'Oh, God, I owe this or that', but then he'd go out and buy an incredibly expensive car. There are different levels of being short of the readies."

Jane must have anticipated that she might be alone one day. "But until it happens, you don't realise how terrifically traumatic it is on the emotions. And it was so sudden. He was so fit and energetic." But he was also a terrific hypochondriac (in his diaries he is forever imagining he has some kind of cancer, or is about to have a heart attack).

In the winter of 1998, his restless energy began to flag. "He had lots of speaking engagements and wouldn't let people down, even when he was running a high temperature with 'flu." Yet all the tests showed that he was fine. By the Chelsea Flower Show in May, he looked terrible, but Jane said: "You've got to be there for the Queen, even if you keel over dead in front of her."

He managed to host a gathering of the XK Car Club, as president, but then the headaches became excruciating. One sleepless night, Jane told him she thought he had a brain tumour. Their GP, "a charming Indian gentleman" they'd never even met before, advised a brain scan.

" 'I'm not going,' said Al. The poor doctor looked at me helplessly and I said: 'He will go and I will take him.'" By the time they reached Ashford, eight miles away, he could not walk. "On the scan, even a lay person could see the tumour, a nightmarish grey mass on the right side."

Once at King's College Hospital he was himself again, "sitting up in bed and shouting at everyone. Frightfully funny, really". After the operation, if anyone asked if he still had his hair he would roar: "Course I have!" He was a predictably mutinous patient.

He would ring Jane at 2am, just to tell her to bring spinach, or Marmite. When word got out about his operation, the press hovered outside, so they evaded them by driving back to Saltwood at midnight.

"Al said, 'I'm never going back into hospital', and I promised him he never would. So the surgeon came here to take out his stitches - they made a sort of pinging noise - and Al was fine, bellowing orders, mowing the lawn, and joining me walking the dogs." He answered a pile of letters by hand, including one from a newspaper editor who wrote: "You can't die yet - Hague's still leader" and one from the Prime Minister.

"We even took a train to Scotland - the most beautiful journey - and were lent a new Discovery Land Rover which I drove, and he said: 'We have to get one of these,' so we did. It is terrifying to think that within a few weeks this man died. You can see from photographs how fit he still looked that summer, striding on the beach with James and Angus." (The four grandsons are all named A. Clark - Angus, Albert, Archie and the one born after Alan's death, Arthur.)

He endured 11 days of travelling to Canterbury daily for radiotherapy. "But the tumour was growing faster than they were killing it. And as he said, the awful thing was, this time he knew what it was. But you still can't believe it. When the marvellous lady came from the hospice, he wouldn't even speak to her. But she knew exactly what pills he needed, so he wasn't in pain. Just overwhelmed with sadness that the miracle wasn't going to happen."

He found music too painful, so Jane would sit on the big bed, reading newspapers aloud. One day she was reading from Matthew Parris and knew she'd got one word quite wrong, but carried on. "And then Al''s voice said, 'Would you just start that bit again?' He was listening, sharp as ever."

He died, with his family around him, on Sunday September 5. He wanted to be buried in the garden, in a shroud, not a coffin. "I'd rejected the first place he chose, because there are bad-tempered bees in the wall and I said: 'I can't sit and talk to you there.' So we decided on this corner, where he can keep an eye on the pool and on me."

He would be pleased to know that he made a beautiful corpse. The face he described as "corroded by misdeeds and self-indulgence" was ageless and unlined. He was 71. They held a family burial service in the ruins of the Norman chapel. So there he lies with the old labrador, Gussy, and Eva Braun the rottweiler.

Jane had looked at the headstones in the stonemason's catalogue, but decided that "Al was a one-off and I wanted the stone to be entirely my choice." She found it in the nearby valley where they were rebuilding the road. A neighbour lugged it over in his JCB. Alan had said he wanted no words on the headstone except his name, his dates and "happily married to Jane for 41 years". He wrote his final will - on a single sheet - in May 1999, leaving everything to her. (Son James was already entrusted with Eriboll, and the Albany set, Clark's London base, was made over to Andrew.) His hoarded and unsorted papers remain in 50 boxes, retrieved from what is now Michael Portillo's office, stacked in the hall.

Saltwood is a huge responsibility to manage. Though she has a housekeeper, Jane still bakes bread and cakes. "But I found it very difficult to make decisions alone, because we shared everything. You feel vulnerable and amazingly insecure. When the moat started leaking, as it did every spring, Al would say: 'Oh God, turn the taps on.' But this year it was doing its usual thing and I told the boys: 'One of your father's cars is going, to re-line the moat.' We caught 600 fish, put them in the swimming pool, pumped out the water - and endless dog balls and children's toys."

I asked if local grandees or gentleman farmers had begun calling to pay court. She shrieked with laughter and said anyone would have a hard time getting past the castle gates, two rottweilers and a moat. "And I have no intention of getting married again. You'd feel sorry for any chap, wouldn't you, who had to follow Al."

She is tougher than anyone imagined. On Millennium Eve she decided she must face the new century alone, and returned from Scotland to walk in the candlelit procession to the village church, then home alone to the sound of fireworks.

"But it's just terrible, this vacancy. It doesn't get better - I burst into tears this morning, having a bath in his bathroom, where he had his last bath. For some reason I found this very upsetting and I decided I've got to change it. But I won't change anything else."

Friends tell her she can't stay shut away here like a medieval lady in a moated grange. "But I've never been on holiday alone in my life. I can't think of anything more dreadful than going off somewhere just to sit. What would you do with yourself? I'd much rather be here, with endless things to do. People say: 'You can do what you want to do now.' But my job was keeping control of things here, in his bolt-hole, where he needed me to be, and I have to go on being head of the house, and run everything as he would want."

I pointed out that he deliberately wrote in what he called "a crabbed hand", just in case a manuscript volume was mislaid. Indeed, later volumes carry the firm injunction on the front cover: REWARD IF FOUND.

Many fans of the published Diaries, though, wondered what all the fuss was about, citing the endpapers printed in the original hardback edition, which reproduce diary entries in an elegant and readable hand. As his publisher for that first volume, maybe this is the place to make a confession. Although the content of those endpapers was taken from actual journals of the period, the reproductions were precisely that. Alan made fair copies, and took considerable pleasure in ensuring that each entry appeared to end with an unresolved cliffhanger.

The question would not, however, go away. When I started work on transcribing the actual manuscript volumes from 1972 onwards, there were moments when I began to wonder if the task might prove impossible after all. The writing was so variable. It seems that much depended on how tired, or how stressed, Alan was when he wrote, as well as the quality of paper on which he was writing, whether he used fountain pen, ballpoint, felt-nib or pencil, and where he did his writing (train journeys to and from his Plymouth constituency did not lend themselves to legibility).

I felt at times that I was learning a new language. Although getting the sense of an entry helped, there were still moments when a whole sentence seemed impossible.

But gradually his writing traits became familiar to me. I found that if one left a difficult passage and returned to it another time, often, as if miraculously, the gist and then the words fell into place. But not always: Jane Clark reassured me that not only did she at times have difficulty reading her husband's hand, so sometimes did he.

The manuscript diaries themselves come in many forms. Early in 1972, Alan was concerned to discover that "those useful, blue cloth-bound plain-sheeted loose-leaf binders", which he had been using since he began keeping a journal in the mid-1950s - what we might today call an A5 ringbinder - were, of course, "no longer being made".

Rummaging around in the library at Saltwood, he turned up a barely used visiting book, bound in crimson-lake leather with the single word "Katoomba" stamped in gold on the front. Katoomba was the name his grandfather gave to his steam yachts, and - particularly pleasing to Alan, who enjoyed the symmetry - the few signatures this volume contained started in the year of his father's birth (1903).

Katoomba - the name came from Australia's Blue Mountains - lasted until 1975, to be followed by a large hardback "legal" notebook. With lined paper, it was identified by him, thanks to its binding, as "the black book". From the 1980s, he used government-issue, hardback A4 notebooks. The paper was appalling - "horrible 'austerity' book", he called the first one - and, on the whole, his writing was worse.

From the earliest days of keeping a diary he usually wrote, once a week, a review of the past seven days. By the 1980s - particularly when Parliament was in session - he was often writing daily, although politics never take over: family, wealth (and lack of it), cars, girls, the countryside, hypochondria and backgammon are rarely ignored for long.

Over the decade covered by this second volume, I estimate that I have drawn from approximately 500,000 words of the original. The style of his writing evolved: there is the note, the simple narrative record and, finally, the more elaborate set-piece, where the sheer pleasure in writing is very apparent. That he kept these diaries for the best part of five decades demonstrates how important they were to him, no matter the pressures.

One mystery was solved only in the final stages of editing the new volume. In Katoomba in February 1974, when he is fighting his first election, Alan refers to an "election diary", but where was it?

I was almost resigned to doing without it. Then Jane Clark rang. No wonder we had missed it: she had uncovered among his papers a WH Smith appointments diary - a page a day - which appeared to contain journal entries. This also became the journal for the October 1974 election, although in the helter-skelter of campaigning, these entries are often little more than brief notes.

On his election to the Commons, to judge from the content, he rarely took Katoomba to Westminster. But the trusty "WH Smith" (being A5 in size and therefore more portable) continued to do occasional double-duty and includes a short account of his maiden speech on April 30, 1974. His parliamentary journal in these early days as an MP is haphazard. He has, for instance, not left us with his thoughts on the overthrowing of Edward Heath, following the Tories' defeat at the second 1974 election, and the arrival of Margaret Thatcher as the Conservative leader. Yet sometimes he felt compelled to seize the minute, writing on House of Commons notepaper before slipping the page into the appropriate spot. The 1975 Common Market debate is one such.

Much was to change in 1979. The general election in May - when he dragged out the WH Smith as his election journal, redating the pages as he went along - saw his majority double, to 11,000, and the Tories returned to government, with Margaret Thatcher as Prime Minister.

Did he always intend to publish selections from his diaries? I believe he was influenced at this point by reading the journal of one of his heroes, the MP "Chips" Channon (the father of Alan's Conservative contemporary, Paul Channon), into which he would dip every day as he drank his early morning tea. He also now realised how much he enjoyed for the first time being of the party in government - even as a backbencher - reporting what he saw, heard and was, increasingly, part of, as well as musing on his own hopes and ambitions. The manuscript book continued, but he also began dictating political entries to his secretary - and to include embryonic footnotes.