"From the start, many of the songs I'm best known for were ballads," he insists. "Look at 'Alison', which is probably the song everybody remembers on the first album. And even the rock songs I did with the Attractions were always more than just the old three-chord trick. I was always a ballad singer who could sing rock 'n' roll."
On the surface it seems a convenient rewriting of history, designed to suit his current purpose: the promotion of his latest album North, a collection of jazzy piano ballads, delivered in a gentle croon over a set of tasteful orchestral arrangements.
"I love to make a noise and I've got a f***-off powerful kind of voice that scares people to hell. It's a loud motherf***er," he says. "But I couldn't do that with the songs on this album. They're very quiet and non-histrionic, and to use that other end of the dynamic range was great. But I don't see why you have to give up one in order to do the other."
He has a point. As early in his career as 1981, Costello took himself off to Nashville to record the country covers album Almost Blue. Since then, there have been albums with the Brodsky Quartet and a jazz project with the Mingus Big Band. Of his last four albums, only last year's When I Was Cruel has been a rock record. Before it came Painted From Memory with the king of easy listening, Burt Bacharach, and For The Stars with the operatic soprano Sofie Von Otter.
Now there's North, and his next release will be a 75-minute orchestral score to a production of A Midsummer Night's Dream by an Italian ballet company, which he's just recorded with the London Symphony Orchestra. Here is clearly a man who can't simply assimilate musical influences, as others do. He has to jump in at the deep end and splash around until he's learned all the strokes and can swim the length of the pool.
It leaves him open to charges of self-indulgence - and not a few critics have suggested that North is an indulgence too far. The crooning and the orchestrations are only half of the complaint. There's also the fact that the album's dozen songs constitute an emotionally naked song cycle that begins with the break-up of Costello's 16-year marriage to Cait O'Riordan, chronicles his subsequent desolation and then joyously celebrates his falling in love with current squeeze, American jazz singer Diana Krall.
"My darling you make everything seem right," he tells the new woman in his life at one point. They are terrific songs. But in such moments, the listener feels almost like a voyeur. It's like reading somebody else's private e-mails.
When we meet for tea, Costello meticulously avoids discussing names. He's dressed in a dark three-piece suit and a sober tie. In his youth, the trademark black-rimmed glasses made him look nerdy. Today in middle-age they make him forbiddingly stern. His initial reaction to my questions is cautious in the extreme.
"Everything you write is in a way personal," he says. "But the morbid consideration of whether the record exactly details my life isn't important." When pushed, he concedes that it is easy to look "ridiculous" when you fall in love, particularly at his age. "But although there's a rapturous expression in some of the songs, they're not happy-go-lucky. They're more about arriving at some kind of peace. I hope people will see something of their own lives reflected in it because it's not exactly unprecedented."
Many writers use the intimate detail of their personal experience to provide the material for their songs. But most use varying degrees of metaphor and allegory. One of the striking qualities of North is its stark directness. Despite his after-the-event coyness, we all know who he is singing about. Did he have no second thoughts about being so transparent?
"Sometimes in the past, for reasons of diplomacy, I've written about intense experiences in a disguised fashion," he says. "There's an impulse to create a little distance and a more impressionistic sense of emotional experience by using irony and word games. But these songs came to me very quickly and demandingly and there's no avoiding their literal definition."
Yet it appears paradoxical to parade your emotions so openly in song and then to regard all further questions about the subject matter as an intrusion. He concedes that people are bound to have a "curiosity", simply because of who he is. But he remains unbending in his refusal to talk about the link between the songs and his personal circumstances.
"Real life is a lot more complicated than songs can ever be. No matter how good they are, songs can't be a literal recitation of life," he insists. Then he comes up with one of those characteristically clever Costello phrases that usually ensures he gets the last word on the subject. Songs are different from the events they may describe because they have to be "crafted into coherence" in a way that real life cannot.
Suitably warned off, I next wonder if Costello feels he has confused his core audience by his sheer, breathtaking diversity and willingness to jump in and out of different styles at will. "The whole question of whether people follow you or not is a question for them, not for me," he replies. "If I allowed myself to think about an audience, then what I do would be a science, not an art. You'd be working on a formula to hypnotise everybody. It would be ‘Let's put a bit of this in to please the Scots' or ‘Let's add something here for the people in Newcastle'. I hate phrases such as ‘demographic' or ‘fan base'. I try not to think about it in those terms because it gets in the way."
He feels that the deep suspicion that Britain harbours towards anyone who dares to step outside of what they're supposed to do is profoundly unhealthy. "It's the ‘who does he think he is?' syndrome, and I've definitely suffered from it," he rails. "It defines our attitude to culture and art. And it's idiotic and self-defeating and offensive."
At last, something has breached the barriers of the urbane politeness he seems to construct around himself these days. And once breached, there's more to come. How does he respond to accusations that he's self-indulgent? "Well of course I am. Because I'm a f***ing artist! That's what I do and it makes you selfish and self-absorbed and I have absolutely no f***ing embarrassment about that at all."
He's finally getting animated, and so I remind him that in the 1980s he wrote "Shipbuilding" as a protest against the Falklands war and "Tramp The Dirt Down," arguably the most bilious anti-Thatcherite diatribe ever penned. Why, at the point when the world seems to be in its most critical state in years, has he apparently chosen to turn his back on political and social comment in favour of the affairs of the heart?
"People assume there's a responsibility to write a song in response to every major event. But it isn't true," he says. "It was easy to react against demonic regimes, like General Pinochet and Margaret Thatcher. But the slipperiness of the current lot makes it more difficult. There will be something to be said in response. But I wasn't moved to do it right now because I didn't want to have to enter their grey world of advertising gobbledygook."
Despite such occasional outbursts, smooth, suave and sophisticated are the words that most readily attach themselves to Costello in his 50th year. So I end by wondering if he was ever really a punk at all, or was it merely a flag of convenience?
"Well, punk had to happen," he replies. "But when cultural annotators talk about the year of punk, it was really the year of disco. In London they might have all been down the Roxy. But in Wakefield they were dancing around their handbags to the Bee Gees."