Mike Leigh has over the years carved himself a very particular niche via a body of films such as Abigail's Party, High Hopes, Naked, Life is Sweet and All or Nothing, works that examine private lives and turmoil with a mixture of unnerving accuracy, tragedy-tinged character comedy and even open parody. Leigh's method of film construction is famously unique: he devises the story and characters (his early films carried the credit "devised by Mike Leigh"), selects his actors and then rehearses with them for weeks on end, allowing them to improvise freely, then constructs the script based in part on these improvisations. When it works best - as in the magnificent Life is Sweet - it produces works that manage to exaggerate reality but still have an extraordinary sense of truth and powerfully connect on an emotional level, and marks Leigh as one of this country's finest film-makers.

So it came as something as a surprise when it was first announced that Leigh's (at that time) latest film was to be not set in suburban London and was not another slice of modern social observation, but a biographical backstage musical set in Victorian times. Could this possibly be a Mike Leigh film? THE Mike Leigh? As it turns out, yes. Leigh has in his time directed extensively for the stage as well as for film, and his clear passion for the theatre comes across here in spades.

Topsy-Turvy revolves around the hugely successful (and influential) partnership of William S. Gilbert and Arthur Sullivan as their latest production, 'Princess Ida', opens at London's Savoy Theatre. Reviews, though favourable, highlight a lack of improvement on their previous works and audiences soon dwindle, not helped by a summer heatwave. Gilbert expresses frustration and Sullivan declares that he will write no more comic operas and instead follow his ambition to write the grand opera he believes the public now expects of him. But a chance visit to a Japanese exhibition in London - which includes a kendo demonstration and a theatrical performance of a scene from The Forty-Seven Ronin - inspires Gilbert, and brings Sullivan back into the fold to work with his long-time collaborator and produce what was to prove one of their very greatest successes, 'The Mikado'.

Biographical films, especially historical ones, are tricky beasts, as there seems to be an almost standard method of presentation and execution that most film-makers seem wearily obliged to follow - at its worst this is stiff, reverential, visually impressive and dull. But just five minutes into Topsy-Turvy you know Leigh has no intention of taking you down that road - even the first shot, of theatre workers meticulously checking the seats for the opening night of the new opera, announces that is not to be your usual period piece.

Where the film does share common ground with Leigh's other work is in its concentration on character over plot, the lively dialogue, the extraordinary performances and the frankly jaw-dropping level of detail. Selling this structure to the uninitiated, especially given the period setting and subject matter, is a job and a half, as the film runs for 152 minutes and the one paragraph plot summary above pretty much covers the essentials - the rest is all character and performance, and we are almost an hour into the film before Gilbert even finds his unexpected inspiration. But therein lies the beauty of the film - in being privy to the private lives of these public figures, in the backstage camaraderie and ego-duels, in following the process from the moment of inspiration to the completed work. Leigh makes it all work so well because he does not in any way sugar-coat the lives he is portraying: Sullivan is struggling with kidney disease and bored with creating the very works that have given him fame; Gilbert is self-centred and relentlessly downbeat about everything ("There's something inherently disappointing about success," he morosely states after the play's triumphant first night); his wife Kitty puts up a front of being devoted and chirpy, but is finding Gilbert's attitude increasingly tiring. Elsewhere Leigh subtly links these issues to the social conditions of the time, so as Kitty uses theatrical metaphor to communicate to her husband her despair at their childless and sexless marriage, Sullivan's mistress Fanny makes her own arrangements for another abortion in a society that scorns children born out of wedlock, a misfortune that ensures lead soprano Leonora Braham is unable to find a male suitor. Her situation has driven her to drink, a vice that is now threatening her career, though lead actor George Grossmith has his own little secret in the shape of a heroin habit.

All of which may make Topsy-Turvy sound like a dark and even depressing tale, but nothing could be further from the truth - these elements are weaved unobtrusively into a film fired with passion and a love for theatre, and one that must rank as Leigh's most uplifting and dare I say joyous work to date. Leigh combines a sly narrative efficiency - Gilbert's moment of inspiration, which has him look directly into the camera and smile knowingly, is followed by an imagined musical number and a scene in which Gilbert is reading his completed script to a clearly delighted Sullivan - with extended scenes in which we just sit back and watch the performers do their jobs. And it's never, ever boring for a single solitary second. Leigh has complete faith in his material and his actors, and neither let him down, resulting in scenes that are an absolute delight to watch, principally for the phenomenal character detail and sheer delight of the acting: Sullivan energetically running though a musical number with his three male leads; the eight minute rehearsal sequence in which Gilbert attempts to iron out some of the eccentricities in the performances of his cast; the hilarious attempts to make the leading ladies walk in 'a Japanese manner', which culminates in a glorious rendition of Three Little Maids From School Are We.... this list goes on and on.

The film's principal pleasure, though, has to be the performances - rarely have I seen a film with such consistently magnificent acting right across the board. Every character leaves their considerable mark, especially in ensemble scenes, where every line is played to the hilt. Selecting individual performances for particular praise is an almost redundant task, but special mention must nevertheless go to Allan Corduner, who is just magnificent as the effervescent Sullivan, though he is given a serious run for his money by Leigh irregular Jim Broadbent as Gilbert, Wendy Nottingham as the no-nonsense Helen Lenoir ("The more I see of men, the more I admire dogs."), Ron Cook as D'Oyly Carte ("Well I don't know about you, but speaking for myself I could murder a pork chop."), Lesley Manville as the resiliently put-upon Kitty, Martin Savage as George Grossmith, Timothy Spall (another Leigh irregular) as Richard Temple and Kevin McKidd as Durward Lely, a Scotsman who feigns an aristocratic English accent when in the theatre but happily drops it when eating out or threatening the costume designer. As if that weren't enough, every one of them holds their own in the energetic stage recreations of Gilbert and Sullivan musical numbers that pepper the film, displaying the same range of talent in those playing the performers as was (and still is) required by stage performers themselves.

Topsy-Turvy may on the surface seem an atypical Mike Leigh film, and in many ways it is, but it is in many ways his most accessible, and still demonstrates many of the qualities that have made Leigh one of the key film-makers of his generation. In the end, though, it comes unreservedly recommended not for who made it, but for what it is, a multi-layered, beautifully performed, joyously exuberant celebration of the world of theatre, and of two of its most famous, talented and fascinating figures.

Sound and Vision