Do you think opera is mostly made up of overweight Italian singers in evening dress, belting out "Nessun dorma" in front of hundreds of wealthy, ageing conservatives, in huge, richly furnished theatres filled with golden statues?

Why do you think that?

If you do think that, you obviously haven't been trawling the London bar scene very diligently over the last few years, because fringe opera, pub opera, or whatever you want to call it, has ballooned of late. It's been amazing in the Bachtrack office seeing more and more small opera companies gain presence on the site, and even more amazing to learn about the incredible range of the events themselves. A Ring Cycle is underway in St. John's Church, Fulham. I went to an opera about Britney Spears in a Peckham warehouse a couple of months ago. And that's just two mad opera projects that we happen to have reviewed. At a time when even the Royal Opera House has new works featuring hoodies and kebab vans, it's pretty clear that opera is enjoying its most comprehensive makeover for a century and a half.

And about time too, according to some. Several of the fringe opera companies I spoke to – not all, mind – spoke of a real sense of disengagement from large-scale opera. William Marsey of ListenPony, an emerging group who recently put on an opera about a woman in love with a chair, says that traditional opera "becomes a closed world", an elite clique for the few. Introducing people to new types of music and new musical experiences is a key prerogative for ListenPony, and especially for frequent collaborator Kate Whitley, an enterprising producer involved in various projects and the composer of the above-mentioned chair opera, Unknown Position. Whitley has been pushing the classical boat out since she was a teenager: her time learning music at the Guildhall Junior Academy contrasted so strikingly to her experiences at a state school, where nobody listened to classical music, that she decided to try and close the gap, bringing classical performances to her schoolfriends. Now, she considers opera to be a better "way in to classical music" than instrumental performance: "The visual element [to music] is very important to how you hear it," she says, and opera therefore gives the audience a clearer starting point.

It's not just about introducing new people to established music, though: she likes to take risks with repertoire, as amply demonstrated by Timberbrit, a piece by American composer Jacob Cooper that Whitley's group Carmen Elektra put on in March. I can testify personally that it was something of a tough listen, containing rather a lot of feedback and yelling, but Whitley is happy to acknowledge this, and unrepentant: "Timberbrit, unpleasant as it was, was not like any other opera. It was a musical experiment," she says. Bringing new audiences to classical music may be on the agenda, but dumbing down is certainly not.

Equally uncompromising in approach is Laura Bowler, Artistic Director of Size Zero Opera, although her aim is not so much to broaden classical music's appeal, as to reclaim opera as a dramatic art. Opera for Bowler is not a tool with which to win people's interest, but an often misrepresented dramatic medium. Coming from a theatre background, she laments that there is "so much focus on the music, that everyone forgets about the theatre" in conventional opera, and she collaborates with theatre directors and actors in order to avoid this trap. Unlike the Carmen Elektra crowd, Size Zero seems quite purely in the modernist tradition, having little concern for making works "accessible" or broadening opera's appeal in particular. Bowler doesn't want to "bribe them to come": her mission is just to produce the best quality work she can. With a performance lined up for this summer's Tête à Tête Opera Festival – long a haven for niche opera groups around the country – and a tour to Singapore in October, she seems to be doing pretty well at it too.

While small-scale opera has been a feature of the capital's cultural life for years – more on this later – it received a particular boost back in 2009, when a pub theatre production of La bohème with a piano and a cast of recent graduates caught the imagination of the press and the public alike. It ran for months, transferring from humble beginnings in the Cock Tavern Theatre to the Soho Theatre, picking up an Olivier Award, and bringing fame to its creators, OperaUpClose. It's really been since then that the stock of small-scale opera has soared high – and maybe the key thing about OperaUpClose's La bohème was its enterprising use of venue. For the opera's second act, which takes place in a bar, the action moved across into an actual bar, giving the staging an added edge of realism that you simply can't get in a traditional opera house. Combined with a rigorous updating of the story, such that it had real contemporary bite, this production was a prominent reminder that opera – even grand, Romantic opera – can actually claim some strange sort of relevance for people today, and young people especially. The show's director and translator, Robin Norton-Hale, is careful not to be too evangelical – "actually about half our audience go to larger opera too," she says – but this Bohème production appears to have provided a spark for imagination in opera production that is changing the way people think about opera overall. OperaUpClose are still going strong, and while they're happy to accept a certain need for conservatism in their repertoire to keep the crowds coming back, their sense of adventure is not restricted to their stagings: they've performed Monteverdi's The Coronation of Poppea, augmented by an extra aria by Michael Nyman, with a jazz trio, and they'll also be tackling Robert Saxton's 1991 opera Caritas this autumn, in their new home, the King's Head Theatre, Islington.

While we're on radical updatings of Romantic classics, though, a burlesque La Traviata surely deserves some points for originality: The Merry Opera Company took just such a show to Upstairs at the Gatehouse, a prominent fringe opera venue in Highgate, with great success earlier this year. Fuelled by the belief that "The classics do have something to say still", this group are big on finding ways to make old works new, and their staged Messiah, set in a church, is quite something as well – it'll be on tour for the third time soon. Their Traviata – and also their upcoming Magic Flute – are aided considerably by brilliant new translations by comedian and writer Kit Hesketh-Harvey. It's the sincerity of their updatings, as well as their genuine humour, which makes their productions so worthwhile.

The Opera Group are perhaps yet more adventurous. Founded in 1997 and with a more tightly professional setup than some of the smaller-scale groups, they combine a bold approach to repertoire with original ideas – like OperaUpClose – in staging. They have also worked with community groups on a number of occasions, including for their production of Kurt Weill's Street Scene, and Human Comedy, an American musical which featured an eighty-strong community chorus. It's very much a non-traditional approach to opera that they preach, and for their upcoming production of Harrison Birtwistle's Bow Down, they are working exclusively with singers who are not trained in opera. The mission isn't just to introduce new audiences to opera, but also new performers – and, partnering with top ensembles such as the London Sinfonietta (for Bow Down) and Southbank Sinfonia, it's a veritable baptism of fire.

One of the relatively few things which unites all of the projects discussed so far is the singers they work with. A vast majority – not surprisingly – are recent music graduates and emerging professional talents. Fringe opera has an important role in giving young singers first professional opportunities, and while it also often a platform for bold new creative ideas, this need not necessarily be the case. Paul Need of Co-Opera Co., for instance, takes a very pragmatic approach to repertoire. "There's no point in doing obscure pieces [the singers] will never do again," he says: the primary point of their productions is to grant young singers performing opportunities, which are becoming all too rare even for those training in conservatoires. Co-Opera make it particularly clear that fringe opera is crucial to young opera singers' careers. Much the same can be said of Hampstead Garden Opera, a group who are also very proud of how their young singers' careers progress. Alastair Macgeorge of the group tells me that "We expect to see at least a half dozen" former HGO singers make it big in bigger opera over the next few years. But as well as this first-rung-on-the-ladder aspect, Macgeorge also emphasises the importance of quality productions to the group, and using as large an ensemble as they can fit Upstairs at the Gatehouse – fifteen players for their recent Così – is a key boast of theirs.

Heritage Opera is another group who make a big impression, mixing a canonic repertoire of Figaros and Carmens with the occasional new piece: Jonathan Dove's Mansfield Park, recently put on by Royal Academy Opera, was a Heritage Opera commission to celebrate their fifth birthday in 2011. Sarah Helsby-Hughes, the artistic director, explains that the group is keen to make opera accessible and dramatically appealing – she's not the only person I spoke to who loathes surtitles – but the company is not primarily concerned with finding new audiences, being perfectly content with drawing an older crowd. "We're quite aware that our core fan base is over the age of 50, and I have no problem with that at all," she says, and so long as new over-50s keep emerging, it's hard to fault her logic as far as I can see. After all, while some might see contemporary opera's key task as finding "new" audiences, this is hardly obligatory, and if people of any age keep coming, this is still cause for celebration. Part of what's so exciting about the fringe scene, after all, is how divergent all the companies' views on opera are.

Virtually every opera company I've spoken to seems to have a completely different set of aesthetics and aims. I've tried to group them above loosely by shared interests, but it's not been easy to do so. And I'm left with several groups which are particularly hard to classify, but whose complete diversity is perhaps indicative in itself of the vibrancy of London's fringe opera scene at the moment. First, Ensemble Serse, a period-instrument group devoted to resurrecting obscure early operas by the likes of Johann Adolf Hasse and Nicola Porpora. Their performances are characterised by impeccable historical research, but specifically not to academic ends: artistic director Calvin Wells says, reasonably enough, that "if these works are to be genuinely reassessed they need to be presented on their, and not our, terms." Expect carefully choreographed recitative realisation, incorporating unorthodox measures and elaborate ornamentation and cadenzas throughout, all in accordance with historical performance practice. Proof, if it were still needed, that patronising audiences is not the way for opera to move forward.

And then there's Opera Erratica, driven by the enterprising director and writer Patrick Eakin Young. He is new to the London scene, having recently moved here from New York, and seems to relish his status as an outsider generally: "I like to see opera," he says, "but I'm not an opera fan; I'm not a musician... I have a certain naïveté about it which I like to maintain." But his personal take on opera leads him down some fascinating creative paths. He describes his recent show Toujours et Près de Moi as a "holographic puppet opera", despite willingly conceding its lack of holographs, puppets and opera. He speaks, unsurprisingly, of having "quite a liberal view of what opera means" – but it's precisely this sort of open-ended, unboxed thinking which is leading the city's operatic revolution.

It would be tough to fault any of these companies for effort, but particular kudos has to go to Fulham Opera, a tiny company who perform in a church, and are now half-way through a Ring Cycle. They don't let the scale of the operation, or the presence of only a piano for accompaniment, detract from some properly Wagnerian singing, and musical director Ben Woodward is rightly proud of the quality they produce. Das Rheingold and Die Walküre are down, and Siegfried and Götterdämmerung are already diary dates for 2013 – as is a complete run-through of all four. Woodward was wondering about Strauss' massive Elektra next. Personally, I reckon they should go for Stockhausen's seven-day, 29-hour cycle Licht, helicopters and all. Clearly, anything is possible in fringe opera these days.

Paul Kilbey 1st June 2012