Steve Coogan looks a bit like Paul Raymond; the two men even share the same adenoidal tic. So Coogan ought to be better placed to play Raymond in a biopic than, say, Meryl Streep was, to play Thatcher. Yet during The Look of Love, it's not the legendary entrepreneur of erotica who fills the screen: it's Alan Partridge.

The character's gestures, mannerisms and intonation hail from the Norwich ring road, not Walker's Court. More importantly, so does his soul. The Look of Love seeks to portray its protagonist as a libertine tragically confounded by doomed paternal love. Coogan's Raymond meets his daughter's demise with an agonised show of remorse; yet his pained expression doesn't have you reaching for your hanky. It makes you think of Alan, ineptly faking his response in some similar situation before being comically caught out.

The problem isn't that Coogan is a bad actor. It's that he's too good. Unfortunately, he's playing the wrong part, the only part he can. He's trying to impersonate Raymond from within the towering comic creation that long ago subsumed him. We can't separate him in our minds from Partridge. Nor can he. Nor even can screenwriter Matt Greenhalgh, who came to The Look of Love after years toiling in the bowels of the British TV culture that Alan bestrode and embodied.

The script can't stop itself from slipping into Partridge mode. "Name one person who has never had a wank," orders Raymond/Partridge. "Mother Teresa," proffers a straight man. "Name another one," fires back our defeated hero, with the forlorn irritability that knows but one parent, who wasn't the sex king of Soho.

It's a pity, because The Look of Love has many virtues, including glittering production design, a great score and an arresting performance from Imogen Poots as the ill-fated daughter. It can't, however, accommodate Alan Partridge.

It's now 22 years since Coogan unleashed his illustrious alter ego on Radio 4. Knowing Me, Knowing You came to television in 1992, and the phenomenon probably peaked with the 2002 series of I'm Alan Partridge. Since then, Coogan has been trying to break free from the gravitational pull of what must seem to him like the Death Star. Escape velocity has not yet been achieved. Something held back his attempts to embody Phileas Fogg, Tony Wilson or The Parole Officer. It's as if, unlike a proper actor, he was all the time conscious of a rival identity. A video released in 2009 encapsulated the sad reality. It was entitled: Steve Coogan Live: As Alan Partridge and Other Less Successful Characters.

This isn't just a case of bog-standard typecasting. Daniel Craig will move on from 007: he's an actor, and for him Bond is just a role. No one thinks he's really Bond or even very much like him. In a sense, however, Coogan really is Partridge. The two are fused into a kind of metacharacter. That's why it's been possible to cast him as a fictional version of himself.

In A Cock and Bull Story, Coogan plays a somewhat Partridge-like character called Steve Coogan. At the time, he told the Guardian that in real life he was more pleasant, more self-deprecating and more intelligent than this personage. Yet watching Coogan on Newsnight, railing against the misdeeds of the media, you could have been watching Partridge.

Michael Winterbottom, who directed A Cock and Bull Story, seems entranced by the crasis of the Coogan screen persona. He toyed directly with the puzzle it poses in The Trip. For whatever reason, he's chosen to intrude one of that film's key components into The Look of Love. Weirdly, the new film's Raymond shares the Observer restaurant reviewer's penchant for mimicry of the stars. Last Tango's Brando, anyone? "Paaaass the Buddha." What's that all about? Is Winterbottom hinting at some essential consanguinity between Coogan/Partridge and the pornographer? If so, it's more than his film can cope with.

All actors must search inside themselves to find glimmerings of the characters they'll depict. Yet some come up with more than mere trace elements: they dredge up their very guts. When this happens, there can be no turning back. Coogan can no more become a Daniel Day-Lewis than Chaplin could play Iron Man, or Benny Hill play Hamlet.

In his bones, Coogan seems to recognise this. His descent into Mid Morning Matters in 2010 marked an acknowledgement that Partridge was bigger than him. In August we're to get the feature version of Partridge that's filled his fans with hope and dread for so long. It may or may not return us to the glories of Partridge's heyday; but Coogan will be back where he belongs.

Back in 2005 he complained: "It's like a band who want to play their new album and people only want to hear their hit singles." Sometimes, however, the people know best.