Alan Partridge's big-screen debut could easily have been a damp squib. A feature film, even one of only 90 minutes, is a very long time to spend in the company of a whining, middle-aged Norfolk DJ with dry skin, athlete's foot and a taste for Fleetwood Mac.

From Porridge to Rising Damp, there are plenty of cautionary examples of bad movies being made from very good British TV comedies. The reason that Alpha Papa breaks this trend isn't simply because of Steve Coogan's prodigiously funny and well-observed turn as Partridge. It's because director Declan Lowney and the film's small army of screenwriters have come up with a Dog Day Afternoon-like comedy-thriller scenario that would just about have worked anyway, even without Partridge at its heart.

The crucial piece of casting here is that of Colm Meaney as Partridge's "friend" and fellow DJ Pat Farrell. Meaney is a formidable actor who makes a perfect straight man for Partridge. He brings menace and pathos to his role without camping it up, even when he is listening to Willie Nelson or driving down Norfolk country roads in a ridiculous yellow radio van. We understand his grievance and believe in his capacity for violence.

Download the new Independent Premium app Sharing the full story, not just the headlines

Partridge and Farrell face the same predicament. They're "old media" dinosaurs, totally out of place in the new digital age. Their station, North Norfolk Digital, has been taken over by new management, Gordale Media, who want to re-brand as "Shape" and cull some of the veteran DJs. After the shake-up, there won't be room for both Partridge and Farrell.

Farrell entreats Partridge to intervene on his behalf. This yields the first great comic set-piece of the film, as Partridge sidles uninvited into a management meeting, pleas for Pat's job and then backtracks rapidly when he realises he stands to be sacked instead. "Just sack Pat" becomes his mantra.

Such shamelessly two-faced behaviour is the essence of Alan Partridge. He is well enough intentioned but the moment his own interests are threatened, he becomes vicious, cowardly and defensive. He may daydream about being a Jason Bourne-like action hero but when the chance comes to seize the gun, he is sure to fluff it.

Pat's response to losing his job is to take all his fellow colleagues hostage. This is where Partridge comes in as chief hostage negotiator.

One of the pleasures of Alpha Papa is the way it shifts from action movie to comedy. Even at the most fraught moments in the siege, Partridge and Farrell still have time to make inane conversation and to broadcast to their listeners. "What is the worst monger? Fish, rumour, iron or war?" is the kind of profound question Partridge likes to set his listeners. He has a sidekick of his own, the ever-cheerful Simon (Tim Key), who keeps his good humour even when wrapped up by Pat in silver tape and with a shotgun pointed at his head through a kitchen-roll holder.

Partridge is a disarming creation. In spite of his casual sexism (most apparent when he is confronted with a woman police boss and can't work out whether to shake her hand or kiss her), his dissembling, narcissism and utter selfishness, we always root for him. With the audience so firmly on his side, it doesn't matter when some of the gags here err a little on the Dick Emery side (for example, Partridge losing his trousers or trying to hide from Pat inside a toilet).

Certain moments wouldn't be out of place in old George Formby or Norman Wisdom comedies. For all the sarcasm and self-conscious jokes about everything from 1970s pop music to action movies, Partridge has an innocent, boyish quality. He also has a quiet desperation about him, as if he knows that if he doesn't act quickly, his career and his status as a minor celebrity will slip away for good.

Rather than try to open up Partridge for an international audience and give him some Austin Powers-like glitz, the film-makers take a hyper-local approach. The finale isn't set in Vegas or Hawaii but on Cromer Pier, out of holiday season. There are no attempts to tone down the British references. Partridge is into his 50s (and Coogan himself is in his late 40s). It's more than 20 years since On the Hour (the radio show that launched Partridge) was first broadcast. It remains to be seen how Alpha Papa will be received by younger British cinemagoers let alone by those abroad. However, given that recent British Film Institute research has revealed over-45s now make up the largest part of the British cinemagoing audience, Partridge is the ideal screen character for his era.

In its own idiosyncratic way, the film deals with topical and poignant issues. There are plenty of real-life equivalents to Gordale Media and its ruthless chief executive Jason Tresswell (Nigel Lindsay), who fetishise youth and see brand awareness as all-important. There are plenty of equivalents, too, to the Pat Farrell-type DJs and their loyal listeners, ignored and discarded because they don't fit with the image of the youth-obsessed brand.

Alpha Papa is hardly the most ambitious piece of film-making. There aren't formal flourishes beyond Partridge's daydream sequences. Much of the action is confined to the North Norfolk Digital studio where the hostages are being held. Even so, the film rattles along at the same steady clip as the stolen yellow broadcast van driving down Norfolk's B roads.

The sly screenplay, with the hostage siege at the centre, provides a storyline with a certain amount of in-built tension while also offering the perfect platform for Partridge as the hostage negotiator. He is the face of the siege, "siege face" as he cheerfully calls himself as his popularity shoots up. It remains to be seen whether this marks the start of a new film franchise but there is clearly life yet in Norwich's finest DJ.