The History Boy who has reinvented the sitcom

So what exactly did he mean? 'I've been through quite a heavy year, professionally and personally and stuff,' he acknowledges softly. But Ruth has always been with him every step of the way. 'What she and I do, and that's perhaps the reason the show is warm, is to try to find what's good in everything. Like people's hearts being broken is horrible and so painful but there are two ways of looking at it. You can think: This is actually quite exciting because eventually someone's going to come along and heal this, and that's going to be amazing. And the muscle in your heart that deals with that will be stronger for dealing with it. God, it's gone a bit morbid,' he deadpans.

Recently, he tells me he turned up at Ruth's house, tired and emotional, and she and her husband, David, fed him and put him up in bed at 10pm.

He met Jones on the set of Fat Friends in 2000, where they first hatched the idea for Gavin & Stacey. Corden had been at a wedding between a girl from Wales and an English guy. 'The only point of reference was that they all knew one of these people who loved each other.'

He and Jones, who is Welsh, began creating the characters but things went on hold in 2004 when he landed The History Boys. Like all the young cast, Corden stayed with the production, which transferred to Broadway and became a film, for three years. 'It was the most special time of my life.' But as they came to the end of the run, Corden realised there was a danger he would get typecast as the comedy fat boy. 'The other guys were having two or three meetings in a day. I just knew I had to take things into my own hands.'

He and Ruth finished writing the pilot and showed it to production company Baby Cow (run by Steve Coogan and Henry Normal), who sold it to BBC3. Despite being newcomers, they got their dream cast, including Alison Steadman as Gavin's pushy mother and Rob Brydon (who went to school with Ruth) as Stacey's sexually ambiguous uncle, Bryn.

Funny, truthful and at times jaw-droppingly subversive, it's the writing that dazzles. Hardly surprising when Jones has worked with the cream of alternative comedy, from Coogan in Saxondale to Julia Davis in Nighty Night (she is also, of course, barmaid Myfanwy, in Little Britain). In turn, Corden has learned from auteurs such as Leigh, Bennett and Shane Meadows - 'people who hold the mirror up and find the extraordinary in the ordinary'.

Winning the Baftas is, he insists, vindication for an oft-criticised TV channel. 'I'm a huge supporter of BBC3, and John Humphrys and Jeremy Paxman talking about how it should be abolished because BBC3 makes TV shows nobody watches is just wrong. I don't understand the point of those guys putting the boot in. It's so foolish. They are not target audience, nor should they be.'

Ouch.

'BBC3 is about giving people the chance to make mistakes without the pressure of ratings. If they get it right, that channel will automatically create the next generation of people who will make shows for BBC1 and BBC2. I don't know how many people watch Newsnight but I feel certain that...no, I don't feel certain at all. I shouldn't say that,' he backpedals furiously. 'But I still recognise that it's an important show.'

Gavin & Stacey is essentially an old-fashioned romantic comedy. But it understands the dynamic of family. Every character is an individual. And it's real: shot in Essex and a tiny house in Barry, South Wales - so you can actually see buses and people go past.

It's got photogenic leads (Mathew Horne who plays Gavin and Joanna Page as Stacey), but it's the show's larger-than-life characters, Nessa (Jones) and Smithy (Corden), who are damn sexy. It's arguably the first TV show where no one apologises about being fat. And everyone is constantly eating. 'I can honestly say I've never felt unattractive,' says Corden, because I've always had interest from women, I guess.'

He was a classic troubled middle child (with older and younger sisters). 'I saw school as one big show-off. I wasn't completely thick but I only cared about English and acting.' He played the class clown and narrowly escaped suspension. 'I wasn't malicious. I just used to mess around.'

An agent spotted him in a play age 12 and he went on the books of the local stage school. But he struggled to find work 'which taught me about rejection from a really early age'.

At 17 he got a role in the West End musical Martin Guerre, where Shane Meadows spotted him for the film 24/7.

It led to a London agent. 'I definitely wouldn't be sitting here now without Shane.' Next came Leigh's All or Nothing about a dysfunctional working-class London family. 'I am so proud of that film. It's beautiful and, yes, bleak but hugely optimistic at the end.'

Corden is visibly proud that his parents are still happily married and that informs his writing. 'Ruth and I grew up in families where love was the important thing.' But his father was away from home for long periods as a musician in the air-force. The uncertainty when he was sent to the Gulf War was especially taxing.

His dad waved goodbye and told Corden he was 'the man of the family now'. 'I just couldn't talk to him on the phone,' he recalls. 'My sisters and my mum could, but I just burst into tears.'

You still catch glimpses of that boy below the surface. He is genuinely 'dumbfounded' by the Baftas victory and close to tears several times. But there's nothing flaky about Corden. He and Jones had a vision, and they've guarded it jealously.

His comedy gods are: The Royle Family, The Office, Peter Kay - and Ronnie Barker, whom Corden part-resembles: a big, handsome man with cheek and subtlety. 'You felt he absolutely knew how good he was but perhaps didn't feel the need to tell everyone.'

It could be Corden's mantra. He's desperate not to become big-headed, 'right now it would be very easy to start to think I'm more of a dude than I am'. Work is still the priority. He and Mathew Horne started writing a BBC3 sketch show today - to be directed by Kathy Burke - and he is writing a film. They allowed themselves a minute of Bafta congratulations, then started work at 10am.

It's been a surreal year for Corden. He's partied with Noel Gallagher. He and Russell Brand watch West Ham together. And then, of course, there's Lily.

He first met Allen when he appeared on her BBC3 chat show last month. 'Let me tell you this,' he told her, 'on my life, without irony or agenda, truly I don't think you realise how lovely you are. And I think you could do with someone to tell you every day.'

Yes, of course, he fancies her. After a very public date, is it friendship or flirtation?

When I switch my tape recorder off, a mortified Corden tells me he had to text Allen to check she didn't think he had boasted they were dating (she's experienced enough in the ways of the media not to get upset).

But if she does decide to snap him up - and it's still a big if - she's got a keeper. Corden understands women.

'Why write about shooting and killing when you can write about love,' he insists? 'And it's happening all the time right now, someone says, I really like you, I love you and someone else says, I love you, too. Let's spend the rest of our lives together if we can,' he says incredulously. 'That's far more interesting to me than people running around with guns, or any Matrix thing. I don't know why anyone needs to make anything up. It's all there.'