James Lees-Milne, diarist and gentleman, has spent 86 years honing the exquisite manners of a lost England
James Lees-Milne laments that, at 86, he has outlived nearly everyone of his own generation. Of his Eton friends and contemporaries from the 1920s, only Sir Rupert Hart-Davis is still alive. Talking to Mr Lees-Milne is like stepping back in time; his manner and conversation give such a vivid impression of the idioms and ideals of the British upper classes in the Thirties and Forties, an age that is nearly as remote to most people as the mid-Victorians.

I telephoned Mr Lees-Milne at quarter to nine in the morning on the day I had arranged to meet him, to make sure that my visit was still convenient. I found him already at work. "It's very wise of you to check," he said, "but the great thing is I am still alive ..." Oh Lord, I thought, and wondered for a moment if I was going to meet a decrepit old man.

Old he may be, but not in the least decrepit. His brain, eyes and hearing are all acute, although he did once lose his way in the middle of an anecdote. Unperturbed, he said, "My thoughts are rather like pigeons in a dovecote: they fly out, but they comeback again." They did, and he finished the story.

His manners are, I was going to write exquisite, but that word suggests archness, a posed perfection of courtesy, whereas Mr Lees-Milne's manners are gentle, considerate, and subtly attentive. When I thought I had lost my gloves, he insisted on retracingour steps to the restaurant and would have climbed the stairs to search for them if I had not forestalled him. His attitude towards women is old-fashioned and protective. He was slightly ill-at-ease in the restaurant at being treated as my guest and embarrassed when I paid the bill, although I assured him I would be reimbursed. "It still feels wrong," he said.

I went to visit James Lees-Milne in Bath because I had so enjoyed his witty and observant diaries and because this year is the centenary of the National Trust, in the decisions of which he played a leading role for nearly 50 years.

When he joined its staff, in 1936, as Secretary to the Historic Buildings Committee, the National Trust had 5,000 members and was administered by about a dozen people, mostly typists. Today it has nearly 2.5 million members and 2,727 salaried staff; lastyear, its shops took £19.5m. Mr Lees-Milne declined to criticise this leviathan, saying only, "I think it's probably a bit unwieldy today but it's an ever-expanding body with a huge number of properties."

He entered the organisation at 26, by a mixture of chance and good contacts, rather than because he was well-qualified, or so at least he claims.

"I was quite untrained, but I had always loved buildings and had been fascinated by architecture. I was very lucky - I got the job through Vita [Sackville-West] whose husband, Harold Nicolson, I knew."

Lees-Milne went on to become an arbiter of style and taste for half a century. We talked in the library on the ground floor of his house in one of Bath's most beautiful crescents, the room in which he does his work. (He still writes about architecture and is considering writing a final volume of diaries.) The building was designed in 1830 by William Beckford for himself and, although the rest of the house was altered long ago, we know from a letter that the library is exactly as Beckford left it when hedied in 1844.

It is a long, harmonious room, lined with glazed bookcases whose wood has mellowed to a deep glossy amber. Hazy sunshine filters through the windows that look across the crescent, gilding the Georgian furniture. On the floor there is a large Persian carpet the colour of old rose, given him by the daughter of Sir Arthur Evans, excavator of Knossos. "It came in mint condition, in a wonderful linen case. On my death it will go to the National Trust, which delights me."

As for his own style, he says it used be thought rather common and effeminate for men to "cut a dash", but he certainly cuts one now. Tall, slim and upright, with snow-white hair, he is wearing a dark brown corduroy jacket whose colour verges on deep purple; under that a raspberry pink shirt and sky-blue V-necked sweater, with a floppy blue and red bow tie.

His diaries have become cult reading, even though the first four are, unaccountably, out of print. Devotees search second-hand bookshops for his beguiling mixture of anecdote, architecture and gossip of an England unrecognisable today. He visited every property in the country worth preserving - many of which have not, however, been preserved - meeting their often eccentric or just plain dotty aristocratic owners. How close is the published diary to the original?

"Oh, the published versions are edited - you simply have to edit them; even Alan Clark admits that! I've tried to be candid yet circumspect. I've destroyed the originals: there didn't seem much point in keeping them." Historians will shudder: Lees-Milne,himself a custodian of the past, destroyed his own diaries? He is unrepentant.

"I wrote my wartime diaries in shorthand on the train, travelling to inspect the Trust's properties. When I started I had no thought of publication; I simply thought I might one day marry and have children and it might be of interest to them to learn what the war was like for a civilian and of the deprivations we went through. It isn't in my nature to expose myself and I had to cut out an awful lot for fear of hurting other people. I am really two generations behind the times, am I not? God knows, I have no illusions about my rather jejune thoughts."

Those "jejune" thoughts can look startling to the modern eye. It may be unfair to tax him with something written nearly 50 years ago, but does he still believe this, from Caves of Ice (the 1946-47 diaries)?

"This evening the whole tragedy of England impressed itself upon me ... A whole social system has now broken down. What will replace it beyond government by the masses, uncultivated, rancorous, savage, philistine; the enemies of all things beautiful? How

Idetest democracy! More and more I believe in benevolent autocracy."

He does not entirely repudiate it. "Of course, when I express these these airy views about democracy it doesn't mean that I think all working people are brutes. I don't like the idea of people being either extremely rich or extremely poor; but I feel that people who are hereditarily rich have a duty to their kind, and it shocks me deeply when I read that heirs to ancient family seats take drugs and behave abominably. The yob culture is everything I dislike. I can't bear their cultural terrorism.

"I've been called a snob. I don't think I am, but I'm certainly an elitist, and I think everybody should be. By elitism I mean the best, the most intelligent, virtuous, amusing or beautiful. I think the squirearchy was a wonderful system, and it producedan awful lot to delight one."

He says that our talk of Victorian values and the belief that family life in those days was stable and all marriages were solid, worthy and dull is misguided. The difference was that people were less likely to analyse their private lives.

Lees-Milne recalls his mother: "She was young and quite uninhibited but she never discussed sex. Only once, I remember, she said to me: `Your father ought to tell you but he probably won't, and anyway sex is disgusting,' adding under her breath, `Not that I find it so!' My mother was very attractive and she had her ... beaux, you know.

"They had their troubles, my parents. We were motoring home once, I was in the back seat, and as we were on the road past Henley my mother was fumbling in the glove pocket and produced a gold cigarette case which had been given by my father to his mistress. She read the inscription and said, `Hm!', opened the window and hurled it on to the grass verge. My father said nothing. Never turned a hair.

"Marriages then were just like marriages now, except that I do think people made more effort to stick together. My parents separated, but they got back together and died on good terms."

Well then, what about love?

"When you look back on those agonies one's gone through for lust - which means nothing - or love! Jealousy is a corroding thing: the only guaranteed way to lose somebody is to show jealousy. It's folly, folly! And you know that, really, seven years is about the limit for passion. The young can't understand that, or don't want to believe it. Passion soon gets burned out, but if you're lucky, love goes on."

If, as he says, he feels two generations out of his time, does he ever regret having lived so long?

"I've traced my father's family back to the 1400s - absolutely undistinguished people, by the way - and they all died young. My father died at the age of 68 and here I am at 86, still going strong. I certainly never expected to reach this age."

His wife Alvilde died last March; they had been happily married for nearly 45 years. "While Alvilde was alive there was some point in being alive because I could help her, but without her I don't think there's much point, really. I wish I could be more certain about the next world. One of the most devout friends I had was John Betjeman. `Hope for the best!' he always said.

"I think the whole pleasure of life lies in anticipation, which is much more exciting and rewarding than fulfilment. There's always the treasure trove of the past for me but the future has becomerather woolly and the present doesn't mean much to me. There's not much to look forward to."

The most recent volume of James Lees-Milne's diary is A Mingled Measure, covering the years 1953-72. The entries show that, whereas he is merciless about houses - describing one as "a detestable little half-timbered atrocity" - he is much kinder about people, always managing to find some redeeming feature. He will describe a difficult dowager as a "noble and splendid woman", or an elderly and querulous aristocrat as "a wise and good old man".

James Lees-Milne possesses one of the rarest and sweetest qualities in any human being: he seems genuinely to like most of the people he meets. When we parted he said (apparently sincerely, but maybe it was just his usual good manners): "I do hope you will come and visit me again - and let me pay for lunch this time!" I do hope he meant it, for I have seldom interviewed anyone whose company I so relished.

`A Mingled Measure' is published by John Murray