What prized possession, the singer Sophie Ellis-Bextor was recently asked, would she grab if she had to abandon her house at a few minutes' notice? "I'd save my photographs first," she replied – so far, so conventional – "then it would have to be my stamp collection."

It is hardly the sort of response you would expect from a woman who has duetted with the Manic Street Preachers and topped both the charts and a readers' poll of "sexy pop icons". Isn't stamp collecting, in the popular stereotype, something for lonely children, adult anoraks or those in the autumn of their years? "I know it's not very rock'n' roll," Ellis-Bextor conceded, "but I've got a big collection."

The 31-year-old "Murder on the Dancefloor" diva is not alone in her unfashionable choice of hobby. Numbers of philatelists (purists argue that the technical term covers both collecting and studying stamps) are on the rise, especially in the hitherto barren territory of thirty- and fortysomething professionals. Stanley Gibbons, the world's leading stamp dealership, reports that booming sales in 2009 (up by a quarter) were largely driven by a new generation of younger collectors coming to their sale-room to pay anything from a couple of hundred pounds for a pristine Penny Black to thousands for rarer examples. And eBay says it has seen a 30 per cent increase in stamp-related transactions.

Tim Hirsch, director of auctions at the stamp specialists Spink, confirms that the demographic is changing. "In the past couple of years, we have seen many more younger buyers than before. What is particularly noteworthy is that many of them didn't even collect stamps as children [known in the trade as 'returners']. These are people being drawn to stamps for the first time."

This new energy in what has until now been seen as a fusty world will be given a wider public echo this month with the revamped London 2010: Festival of Stamps. This once-a-decade event, co-ordinated by the British Postal Museum & Archive, is keen to shed its image as an in-house get-together for a hardcore minority. "Stamp collecting is undoubtedly going through a period of transition," says Jennifer Flippance, exhibitions and project manager for the festival. "In the 1960s and 1970s, it was something lots of children did, but then it began to be seen as a bit nerdy, and numbers dropped. People began talking about it dying out. Now, though, we hope the various exhibitions we are staging throughout the year will see stamp collecting come out the other side as something as popular, respectable and even fashionable as collecting art. Stamps are, after all, the world's biggest public art gallery. They provide, in miniature, collectable pictures of a changing world that everyone can afford."

The highlight of the 2010 programme is Empire Mail: George V and the GPO, an exhibition of stamps and artefacts from the era of the present Queen's grandfather at the Guildhall Art Gallery in the City of London. There are also walking tours exploring postal history, specialist exhibits of themed stamps at Wimbledon Lawn Tennis Museum and Bath Postal Museum, and eye-catching displays from private collections at the Business Design Centre in Islington that aim to make stamps more accessible as public art.

So what is transforming stamp collecting from something that even its estimated 2.5 million devotees were slightly ashamed to own up to in public, to being, as one new convert puts it, "the new knitting"? Hirsch suggests two reasons. The first is financial – and another consequence of the global economic meltdown. "As collectables," he says, "stamps have continued to show steady, robust increases in value, year on year, without violent swings. So in these uncertain times, they are undeniably attractive to investors."

Statistics bear him out. Stanley Gibbons operates its own equivalent of the FTSE 100 index to track the trade in collectable stamps. It has risen 89.7 per cent since 2000. There is also the more specialist GB30 Rarities Index – reflecting the values of more highly prized specimens. (Spink's latest catalogue features items with a reserve price of £40,000, while a world record $3.8m was achieved in 1993 for a pair of extremely rare 1847 stamps from Mauritius.) This index rose by 39 per cent in 2008 alone, the very year global stock markets were crashing.

The second factor bringing new life to what had been a stagnant and ageing market, Hirsch feels, is a rediscovery of the intricate aesthetics of stamps. That is precisely what appeals to one of the new breed of collectors, 40-year-old author and this newspaper's gardening expert, Emma Townshend. "When I'm in America," she says, "I always find myself buying loads of stamps because they produce such beautiful examples, like whole sets of pictures of their national parks. I like the look of them so much that I do what proper collectors always say you shouldn't do with stamps – I use them to decorate the covers of diaries and notebooks."

Townshend began collecting early – partly harvesting stamps off envelopes in the traditional schoolgirl way, though her album was boosted because her father, Pete Townshend of The Who, received fan letters from all around the globe. She also inherited a collection from one of her grandmother's lodgers. "He and I shared a birthday, though he was 40 years older than me, and he gave me his album, which even then I knew was properly valuable." And she has been collecting off and on ever since. "It is something that comes and goes in fits and starts." The stamps that continue to draw her back in are the ones that make the most visual impact. 'They stand out so clearly. There was a Royal Mail Christmas set in 1973, I think, with King Wenceslas that is just so gorgeous. I still enjoy taking it out and looking at it."

Businessman Simon Martin-Redman's take on philately is rather different. Like Townshend, this 53-year-old from Northamptonshire (pictured left) began collecting when he was a child – "I have a photograph of me, aged two, holding an album, which must be a world record." After running a successful political-lobbying firm and then a management consultancy, he has found himself over the past five years devoting more and more time – and money – to collecting stamps, and in particular those issued in Sarawak, once a semi-detached part of the British Empire run by "White Rajahs", and now part of Malaysia.

Part and parcel of stamp collecting is an interest in detail. Many enthusiasts have a specialist area – anything from a single, possibly otherwise obscure, country as in the case of Martin-Redman, to designs featuring particular animals, vehicles or famous faces. Former world chess champion Anatoly Karpov, for example, collects those depicting his sport.

There is also the search among what are usually mass-produced products for individual items whose anomalies make them stand out – such as the 2d Tyrian Plum that is part of the Guildhall exhibition. Though 100,000 sheets were prepared, only one ever went through the postal system. It was stuck on an envelope to the then-Prince of Wales on 5 May 1910. By the time the letter arrived the next day, his father, Edward VII, had died, George was now king, and the entire print run of Tyrian Plums was destroyed rather than released to the public because it did not feature the new monarch's head.

In a survey carried out among enthusiasts by the organisers of the 2010 festival, 31 per cent of respondents said that "hunting out items for my collection and the sense of satisfaction from that" was their favourite aspect of their hobby, compared with 18 per cent drawn to the beauty of stamps and 17 per cent to their investment potential.

That "hunting out" can be, Martin-Redman admits candidly, "a form of intellectual masturbation, all about the self-gratification of the individual", but he adds another element to the mix – the competitive edge. "I am by nature an extremely competitive individual and therefore want to be best of breed in my specialism. I was determined to build up the best collection of Sarawak stamps." Though he is reluctant to put a figure on how much he has spent – "for me it has been the equivalent of others collecting art or fine wine, the investment potential and the pleasure derived cannot be separated" – a measure of his single-mindedness is that he is now one of only three private British collectors in the exclusive Club de Monte Carlo, restricted to the world's top 100 philatelists.

Is its composition changing to reflect the current revival? "Well, it certainly doesn't conform to the old image of stamp collectors," he replies. "That was horribly seedy. I do seriously see philatelists now as doing the equivalent of coming out of the closet, casting off that fear of being exposed in public for doing something they ought to be ashamed of. It is now a very sociable world, and whenever we meet in Monaco, there are grand dinners and cocktail parties which are attended not just by philatelists but by what you might call high society."

The small talk at such gatherings must inevitably include the changing face of stamp collecting. "Someone told me recently," confirms Martin-Redman, "and I cannot provide empirical evidence for this, only repeat his remark – that philately is currently the fasting-growing hobby in America for professional males over 40, and that the reason for this is that they are fed up with spending their lives in front of screens, whether they be computers at work, or TVs and monitors when they get home, and so are being drawn to something older, more enduring and more hands-on."

This appeal of stamps as part of a return to a purer, less technology-obsessed age has also been noticed by Jennifer Flippance. "There is a wider movement going on right now about returning to values of thrift, make-do-and-mend, deriving pleasure from doing simple, manual things rather than spending ever-larger amounts on technology which quickly becomes outdated. Stamp collecting feeds into that. Indeed, you could argue that it is because it is so old-school that it can now be classified as cool."

For some, though, stamp collecting has an appeal that goes much deeper than any passing social trend. The writer Simon Garfield explored in his 2008 memoir, The Error World, the connection between his own return, in his forties, to philately (specialising in stamps with errors in their printing) with a midlife crisis that saw the breakdown of his marriage.

"I'd first been interested in stamps," he recalls, "when I was seven or eight in the 1960s, which was a time when there wasn't very much foreign travel and we were generally less aware of the outside world. So back then it enabled me to learn a lot, however swottish that sounds. It was certainly never the hip thing to do. But a part of it was always that I have the collecting gene. I couldn't help myself. And that is what resurfaced in my forties, though now I had money in my pocket and so I found myself, secretly, spending amounts that I am in retrospect ashamed of. But there is a way in which collecting fills a hole in a life, gives it a semblance of meaning. Owning a piece of history – however common, however rare – may even create a fleeting purpose in this world. For me, certainly, getting involved in stamp collecting again was part of going through a difficult emotional patch."

It was a different sort of emotional journey that brought 58-year-old design guru Stephen Bayley back to stamps. He was clearing out his parents' house after their deaths when he rediscovered his childhood stamp albums and re-engaged with them. He sees the crucial factor in today's revival in interest in stamps as neither financial nor aesthetic. "It is the elegiac aspect that is important," he stresses. "For some there is a nostalgia for their own past, but more widely people are realising that stamps are not likely to be with us for very much longer. They are rather like other minor art forms – such as ashtrays in pubs – that are soon to be lost. That realisation generates a wish to collect them, to preserve them as part of a disappearing culture."

Stamps do, he concedes, have rather more to recommend them than ashtrays. "They involve a whole range of creativity, within clear disciplines, not least dictated by their size. So by collecting stamps, you are, at a fraction of the cost of collecting other forms of art, gaining access to a vast international archive of design."

How does this style expert and social commentator rate stamp collecting's long-term chances of losing the stigma of being on a par with twitching or model-airplane making? "It's possible," Bayley says. "If I was given the brief by an international advertising agency to make stamps fashionable, my approach would be to present them first and foremost as an opportunity, open to all, to build an archive of amazing international graphic design."

For details of the Festival of Stamps, visit london2010.org.uk

Stamps and glue and rock'n'roll

The Rolling Stones guitarist Ronnie Wood is a prominent recent convert to stamp collecting. He took it up on coming out of rehab as a way of "trying to stay on the straight and narrow".

Tennis champion Maria Sharapova started collecting when she was a child, but has kept it up into adulthood. "I have been lucky to travel to some amazing countries and I always try to collect stamps from every place I go."

And French president Nicolas Sarkozy, a keen philatelist, sees the hobby as "an opening to the world, to history, to great events... and to a world of artists, engravers and page designers".

Other noted collectors have included John Lennon – his childhood album, complete with his sketches on some of the stamps, is at the Smithsonian National Postal Museum in Washington DC.