The shows must go on, and on...

The one disappointment is that the geraniums don't burst into song. Doubtless because Sir Cameron Mackintosh judges them more effective as they are, dribbling deliciously down the only double-fronted house in this particular Bloomsbury square. Behind them is his powerhouse, an Adam interior of domed ceilings and alcoves, fancy plasterwork, marbled floors and - just a teensy bit camp, this - a blue plaque saying "Cameron Mackintosh flourishes here: 1988- ". Till forever probably.

Picture the crowd at Wembley last week, then marvel at the fact that almost the same number (68,000 to be exact) see a Cameron Mackintosh show somewhere in the world every night. His producing company alone turns over pounds 40 million a year and makes profits of pounds 11m; the salaries of its three directors total pounds 7.3m and it's fair to say that Mackintosh gets the lion's share. Personally, he's worth an estimated pounds 250m and last year was ranked 55th richest man in Britain. And so, in the same week that Sir Andrew Lloyd Webber was belly-aching publicly about the "parlous" state of the British musical, Mackintosh was cheerfully telling me that "it's a huge growth industry and one we do better than anyone else in the world". Which nicely sums up the difference between the two great musical impresarios. While Lloyd Webber's cup is always half empty, Mackintosh's is always half full. If Mackintosh didn't relish every aspect of his job, he could have hung up his cloak when Cats made him his first few million - but neither the cloak nor the idleness are his style.

Exactly what that style is proves hard to define. This is a man who shares his office with a collection of teddy bears (which, sadly, I won't be meeting because I'm told to go instead to the theatre where the gaffer is fiddling round with a Martin Guerre run-through). This is a man who, aged eight, was spellbound by Salad Days and knew from that moment on that he was destined to make a spectacle for himself. "You've got more dying relatives than the whole of the West of Ireland," observed his house- master, Brother Keegan, at the Catholic public school he attended when bumptious young Mackintosh invented yet another tragedy to get leave to bunk off to a new show. As producer of the school revue, he started selling tickets on the first day of term, discounting all advance bookings to pay the rent on the lighting equipment. Arithmetic was his strong suit, he says, but theorems stumped him. "I've got a very practical brain. As for the eye and ear thing, you'll have to ask God that."

It's easy to see how he charmed Brother Keegan and sweet-talked pounds 25 out of his mother to invest in each show in those impecunious early years when he had to get down to Lisson Grove dole office before his actors queued up for theirs, when cheques bounced, scenery fell down and the leading lady ran away. "And I thought it was all normal. If I'd had any knowledge, I would have run away myself." A chubby bundle of irrepressible enthusiasm, utterly unspoilt by good fortune, he's hitting 50 but has the perennial youthfulness that so often attends the rich and successful. He's, well, teddybearish - cuddly, adorable and adored. And dig as deep as you can and there's no dirty linen (he's been cosily tucked up with his photographer friend Michael Le Poer Trench for 15 years and more), no dodgy financial deals, none of the prickly intellectual insecurity or desperate desire to be taken seriously that torments his arch-rival and friend, Sir Andrew Lloyd Webber.

Still, working for Mackintosh is no holiday. He has a reputation for interfering massively and obsessively, for getting those podgy fingers into every aspect of making mega-musicals. While he can't sing or play an instrument, claims his sense of timing is appalling, and admits he barely knows his Bach from his Beethoven, he'll deliberate upon every grace note of a score and insist on changes if he thinks "there's a button needed here or there or there's a bit of music in the wrong place or the orchestration doesn't have the right dramatic feel". Directors, designers, choreographers, actors, even front-of-house staff - all can expect similar attention and instruction. Maybe it's the way he dishes it out, but nobody seems to resent him for it. Indeed, it appears that they trust his judgement ("I call it instinct") better than their own, which is hardly surprising given his hit rate. At the last count, Cats had turned over pounds 1 billion, Les Mis pounds 750 million, Phantom pounds 1.4bn, Miss Saigon pounds 500m. Everywhere the shows play to over 100 per cent capacity (ie the standing room is all taken up). When it comes to tuning in to popular taste, Cameron Mackintosh has near-perfect pitch.

It took a bit of time for Mackintosh to get into the big league, but when it happened - thanks to a pivotal collaboration with Andrew Lloyd Webber on Cats - a phenomenon was born. This innovatory show, staged by people who apparently cared about its literary roots as much as its physical spectacle, set the tone for the future. Arguably, from this point on, the British theatre became internationally famous not for the quality of its new writing but for its outstanding ability to stage spectacular musicals. Even stars weren't really necessary any more, the show itself hogged the limelight. Last year, musicals made up 62 per cent of all West End theatre attendances, while new drama drew 11 per cent, revivals only eight. Of 19 musicals now running, the five with the queues furthest round the block are Mackintosh's.

Will Martin Guerre bring him the same success? Claude-Michel Schonberg and Alain Boublil - who are reputedly coining pounds 20,000 a week for their contribution to Les Mis - first came to him with the idea six years ago. He didn't like it. "It didn't have the sweep that is their trademark." He heard it again in 1993 and didn't like it any better: "There were terrific nuggets of Boublil and Schonberg songs in it but it hadn't come together." He sat down and explained why he wouldn't be getting involved. "It was one of those extraordinary days. We sat down at 11 o'clock and by the end we'd discovered the integral part of the show. I realised that everything was there and it was just a matter of threading it all together."

It's still taken some doing. First, Boublil and Schonberg were late with the product and Declan Donellan, the director, was left to occupy his cast with potato-picking workshops; then, even after previews began a couple of weeks ago, Mackintosh still spent the days "fiddling round, working on the clarity, the pacing". A new lyricist, Edward Hardy, a graduate of Sondheim's masterclasses, took over from Herbie Kretzmer and, according to Mackintosh, has given the show the "sophisticated but rough-hewn quality" he says it needed.

"I'm terribly pleased with how it's going," he admits, suddenly consumed with a snuffling, rather embarrassed and childlike excitement. He starts gushing about the "naturalistic sound - I don't think anyone will know it's amplified" and the wonderful set, five structures swivelling like a vertical kaleidoscope, and the trees - "copies of real trees from Epping Forest". It may be a show about peasants, but Mackintosh's idea of rustic doesn't mean hick. Some previewers have called it a Les Mis rehash (not surprising, given the Schonberg/Boublil factor, and not necessarily a criticism), others emerged a little hazy about what had actually happened, claiming it wasn't the story - basically, mystery man returns from war to reclaim wife and property - they thought they knew from the Gerard Depardieu film.

Mackintosh says he wasn't wild about the movie: he disliked "the selfish, calculating character" Depardieu played. "You can only care about what happens to characters if you like them." He didn't want any flashbacks, "which I don't believe belong in a musical". He knows what he likes and his wishes become commands very early on. Unlike most impresarios, he doesn't merely assemble a team, then find the money for them to create the show. "I'm convinced that, with most story shows, authors should create first. The director and choreographer shouldn't be brought in before there's something to root out." Nor does he continually recycle the same tried and tested talents. Just as he knew Miss Saigon needed Nick Hytner's unsentimental, astringent treatment, so he believes that Martin Guerre will be best delivered by Cheek by Jowl's director-designer duo, Declan Donellan and Nick Ormerod. "Having seen their Fuente Ovejuna and Peer Gynt, I felt that peasants is what they're good at."

Fairy-godmothers don't come sweeter than Mackintosh. Little by little he is masterminding a redistribution of theatrical wealth. Even his snootiest, most cynical critics acknowledge that, for every new Mackintosh mega-musical clogging up a West End theatre, there is a delightful flip-side in the spectacle of a relatively poverty-stricken director previously committed to the subsidised theatre made rich beyond his (so far they have all been male) wildest dreams. The rumour is that it's Stephen Daldry's turn next with Mary Poppins. Mackintosh denies patronage and calls it pragmatism - "I look for the best possible talents and harness them to the right projects, which quite deservedly make these people rich" - but his generous support, direct and indirect, of the subsidised theatre puts an extra sparkle on his halo. He endowed a chair of musical drama at Oxford and installed Stephen Sondheim as first incumbent. One of the reasons why he continues to use a very select band of backers for his shows, rather than spend all his own money, is because he believes that the money they make on his shows will be reinvested elsewhere in the theatre. "The reason I have these fantastic talents working for me at the moment is that they've been trained in the subsidised theatre. If you care about the longterm, you've got to keep ploughing back into the theatre generally."

He has little time for those detractors who accuse him of devaluing the musical theatre, of fobbing off an audience with spectacle and industrial technology where there was once wit and style, of replacing the book musical with a saggy sung-through job. "The fact is that Andrew [Lloyd Webber], Tim [Rice], and Claude-Michel and Alain are only happy writing in that form. It's a bit like saying, 'Why doesn't Shaw write like Shakespeare?' As for spectacle, compared to what our great-great-grandfathers used to do, what we're putting on is Marcel Marceau having lost his set."

n 'Martin Guerre' opens at the Prince Edward Theatre, London W1 on Wednesday. Booking: 0171-434 8951