There is nothing wrong with being self-invented. The most interesting people in the world designed themselves. And in this matter Roy Strong, once upon a time the director of the Victoria & Albert Museum and National Portrait Gallery, can offer a master class. He has discovered the mines of self-invention to be very deep and richly seamed with treasure. This is no less than his third bulky volume of diaries, and readers have been generously treated to autobiographies as well.

While convinced that a scheming Alan Yentob conspired to keep him off the telly for more than 30 years, Roy, with his singular voice, is a national asset, recognisable from innumerable radio broadcasts. With great art he has retained a bit of a suburban twang from Edmonton Grammar School, but presents overall as posh. The ever-so-slightly camp lisp is a nice touch, suggestive of self-deprecation where, I suspect, not a lot of that exists.

A brilliant university career removed him from ‘awful, humiliating’ suburban mediocrity. His father was a disobliging and artless hat salesman. Mum was Mabel. Elizabethan court masques became a special subject. This is a genre of exaggerated histrionics, of dressing up and striking mannered poses. Who says academic research is no preparation for life?

But I don’t want to be a bitch. This diarist is sometimes bitchy with himself. I always admired Strong from afar. With flares, kipper ties, stylised facial hair and fedora he became a swinging director of the NPG at just 32. If anyone can claim to have modernised museums it was Roy. He admirably advised that visitors should be able to enjoy ‘martinis with their Bellinis’. Private Eye’s Pseuds’ Corner once published a photograph of him holding a champagne glass. No quotation: just the picture.

At the V&A Strong had to decide whether to mend a leaking roof or make the creaking institution a media phenomenon. He successfully chose the latter, but alienated the retardataire keepers who construed the museum to be a connection of private feudal baronies. They regarded Strong with all the distaste you would show to a dead rat on a stick. When he left it was made clear by officials that he would not be employed in public service again. This still wounds.

That was in 1987. Thus these new diaries are in the long-deposed-emperor mode, and they come with an autumnal feel, although the score-settling remains brightly and impressively energetic. Strong’s immediate successors as directors get not even a retrospective mention here, although he approves of the lacklustre tenure of Sir Mark Jones, who invited him back — a gesture he admired. On that occasion Roy was introduced to a woman who astonished because she had ‘never heard of me’. His revenge is to stamp his pantofle and describe her as ‘boring’.

It was in the context of simmering internal revolt, of betrayal and plotting, that I met Roy. With a winning mixture of generosity and opportunism he invited Terence Conran and me to get our Design Museum project started in his hitherto neglected basement. This allowed him to claim fashionable modernist tendencies, while at the same time having nothing at all to do with them. Apart, that is, from cultivating Gianni Versace.

I enjoyed his wit and style, and we shared many joshing lunches. I even showed him how to work the Amstrad computer that was troubling him at The Laskett, his bosky country house. His relations with Conran were less happy. In fact, he detested him with relish.

Perhaps Roy was jealous of Terence’s money. In 1976 he had reviewed the Habitat catalogue, describing one model as looking like a ‘1940s tart’ (something Terence would have enjoyed) and snootily said a room-set looked like a ‘Hendon semi’ (he knew whereof he spoke). Terence’s riposte to an accumulation of antagonism was to suggest that Roy should be stuffed and put on display in his own museum.

A schism in our culture was revealed in the emotional gap between these two remarkable men. Roy thought Terence tactless, abrasive, egotistical, misogynistic and bullying. Fair enough. Terence thought Roy a bit of a twerp. One day a flustered Roy rushed into my subterranean office to denounce Terence for having had the oafish effrontery to sniff and handle a medieval manuscript. ‘How can you work for that man?’ he wanted to know, almost sobbing.

The diaries are revelatory of a man of taste approaching the foothills of the Annapurna of social mountaineering. The cast of characters, designed to impress, runs to 70 sonorous pages at the back of the book. As a diarist, reference points were surely Bernard Berenson (superlative aesthete) or James Lees-Milne (acidulous snob, whose formulaic titles have been inspirational), but on this occasion Roy more nearly approaches Sasha Swire and Barbara Amiel. It’s the Mail Online rather than the Burlington Magazine.

There is much to enjoy. Simon Schama, a Yentob client, we are told, cannot tell the back of a painting from its front. The Schama telly persona is ‘simply awful’. But how curious to have no reference to Norman Foster — or to poor Terence Conran, whose declining years these were too. Nor to any art critic, not even John Richardson or Umberto Eco. But Elton John and Cilla Black make the cut. And a Pope.

The signal event of this long, long, long volume is the failure, after years of campaigning, to persuade the National Trust to accept his mollycoddled garden, an impressive but overwrought and over-thought undertaking. One of the keepers he had antagonised described it cruelly as ‘Mr Pooter’s Versailles’. The National Trust simply thought it was not important enough, another judgment that stung.

Still, there was support from a version of society. No one has ever enjoyed an engraved stiffy from a nob more than Roy. Until he bequeathed his archive to the Bodleian, his house was rammed with Solander boxes, files, scrapbooks, photo albums, commonplace books and diaries containing, for all I know, every teenage bus ticket he ever acquired shuttling between Winchmore Hill and Edmonton. His validation by ephemera may be as disturbed as my own craving for by lines.

Readers not aware of Roy’s distinguished past may find these diaries deranged in their self-regard: it is a rare Strong-branded lecture that is less than a ‘sell out’. But soon you realise they are very funny and endearing in their pathos. Yet there is the occasional peevish lack of generosity. When interviewed about the V&A’s contribution to an understanding of modern design, Roy somehow forgot to mention Conran and me.

With Roy now 85, we may assume this will be the last volume. But there is an image that will last forever. For an 80th birthday exhibition, he travelled the country (‘motored’ he would say) with the photographer John Swannell: ‘We set off for Dorset at 10 a.m. on a glorious sunny day, with a car packed with chain mail and armour.’ The result was a sequence of bizarre photographs of our verray parfit gentil knight got up as Tennyson, Brunel, a maharajah, Henry VIII and Federigo da Montefeltro (in the Piero version). Surely this was most peculiar?

After 500 pages, I was increasingly uncertain what world these diaries recorded. But of one thing I was sure: Dr Sir Roy Strong CH, now additionally a Searcher of the Sanctuary at Westminster Abbey, is a unique, admirable and enduring invention. More true, in fact, than his temporary postures.