You'd expect an Alan Partridge movie to be funny. You wouldn't expect it to be a coup de cinema. And yet, in its own superficially low-brow, cheapo way, Alpha Papa is exactly that. It's a Fabergé egg disguised as a Kinder Surprise; an intricate piece of engineering done up to look like a whoopee cushion.

Big-screen Partridge has been chatted about for a decade now, and concerns have stayed the same. Surely Alan's petty parochialism isn't cinematic catnip? Wouldn't such a radical shift of situation endanger the comedy?

But in his 21-year career, Alan has already proved unexpectedly flexible. He's straddled radio, TV, stage, print, online, news spoof, chat show, memoir, mockumentary, meta-farce. This quantity of content comes with a downside, of course – many fans are so invested they feel they know Partridge better than his creators. Mess with the mythology at your peril.

The genius of Alpha Papa, then, is in remaining faithful to Partridge's small-screen soul while also managing the demands of a big-screen Alan. Movies need an arc, preferably a redemptive one – and what Alpha Papa offers is Partridge's plausible reinvention as a real hero. One whose conscience is awakened through self-interest, perhaps, but a hero nonetheless.

We first re-meet Alan hosting the mid-morning spot on North Norfolk Digital radio. He's content enough, canvassing on-air opinion on the best "monger" (iron? fish? war?) and rebuking sidekick Simon for misjudged banter ("Never criticise Muslims. Only Christians. And Jews, a little bit").

But an evil media conglomerate has taken over the station and fires one of the DJs, night-shift Irishman Pat Farrell (Colm Meaney). Pat retaliates by staging a siege at the office party, taking hostages and refusing to negotiate with anyone save Partridge, who is duly despatched by the police.

The Alan we have here is an Alan in shape for action. If not actively attractive, he's not objectively repellent, with his glossy demi-mullet and none-too-tubby physique. Most notably, his skin lacks the horrid mottle of many TV adventures; and though the decision to skip the ageing makeup (Coogan is 47 to Partridge's 55) is initially disconcerting, that grotesque element might have glared a bit in widescreen.

Father Ted veteran Declan Lowney directs with one eye on the bigger picture – glorious panoramas of Cromer - and another on the familiar. The majority is shot on handheld, as pioneered by the Travel Tavern series, which gives freest rein possible to Coogan, on very nimble form. There's some lovely slapstick here, terrific skipping, even a touch of Leonard Rossiter – that rat-tat-tat delivery, the guilty flick of eyes to the side.

Yet it's also true that one keynote moment of physical comedy – in which Alan's slacks get snagged on a window sill – is not really specific to Partridge. At a push, this is standard-issue icky business which could be air-lifted to a different film entirely. Likewise, his report on a romantic encounter in disabled loo is hilarious, but not really because it involves Alan.

There are elements of Alpha Papa which could have arisen more organically. Rather than making the DJ who goes rogue a cast addition – and not an enormously interesting one – perhaps a returning character such as raddled alcoholic Dave Clifton (Phil Cornwell) could have flipped instead. And the pleasures of Partridge are still most intense at their most mundane: lip-syncing in the motor, boring on about bovine gynaecology.

Coogan and co often credit Partridge's appeal to the idea that Alan is us, unchecked; that he expresses what we'd all say if we didn't self-edit. Alan has no filter because he doesn't perceive the need for one (unlike, say, Larry David, who's similarly frank, but with confidence). It's Alan's naiveté which explains the affection in which he's held, and why we root for him.

And it's this which the film takes a risk with: are we prepared to back Alan even when he exhibits signs of self-awareness? I think the gamble comes off; that Alpha Papa is the Partridge equivalent of what happens to David Brent in the closing episode of The Office: a vindication, even a dignification.

Alan must be knocked down again, of course, if he's to continue. But Alpha Papa, in allowing him the kind of starring vehicle Partridge himself might enjoy, feels like a permissible victory. For a character to spend 21 years being a loser on telly is a comedy. That he'd also lose on film might have felt like a tragedy.

• Alan Partridge: Alpha Papa is released in the UK on 7 August

• A news report on Alpha Papa's Norwich premiere, and a gallery of Alan on the red carpet

• Win an overnight stay at a four-star hotel in Norwich plus dinner, breakfast and a Visitor Attractions Pass for the city