When Doctor Who Magazine polled its readers to name their all-time favourite Doctor, David Tennant won by a fair few votes, dislodging Tom Baker for the first time in years. Now the BBC has also thrown its weight behind Tennant, signing him up until 2008 for £1m, making him the most expensive Time Lord in the show's 30-year history.

Some might suggest that a million pounds is not enough to compensate for spending nine months of the year in Cardiff being chased by homicidal pepperpots, but Tennant has proved himself to be an exuberant, charismatic 10th Doctor. Like Baker, Tennant fills the Doctor's shoes with a quirky sense of style and humour, perhaps more so than predecessor Christopher Eccleston.

Landing the role of one of TV's most iconic characters has propelled Tennant into the big time, but the series is also the reason he went into acting. Aged three, he told his parents that he was going to be an actor and play Doctor Who. "It has been a lifelong dream to get my own Tardis," he says.

Born in Bathgate, Tennant has also done his own bit of identity-shifting, changing his name from David McDonald to avoid clashing with another actor. He adopted the surname of his favourite Pet Shop Boy, Neil Tennant.

Tennant was on screen before he was out of Paisley Grammar, talent-spotted by Scottish TV at a Saturday youth theatre club, an offshoot of the Royal Scottish Academy of Music and Drama where he later trained. Not all the reviews were positive, he recalls: "I was playing King Arthur in Edinburgh, which was only my second job, and the review in the Scotsman said: 'The cast of 18 are uniformly excellent, with the exception of David Tennant, who lacks any charm or ability whatsoever.' Which I have to say floored me for quite a while."

In his 20s, he joined 7:84, the angry young theatre company whose work made Das Kapital look like David Cameron's desk diary. Tennant wasn't particularly into the politics, he says, just the employment.

The thing that he says "changed my life" was a lead role in the 1994 TV drama Takin' Over The Asylum, which he won after spending one day on another drama called Strathblair. It was enough to impress the director, David Blair, who cast him as manic-depressive radio station manager Campbell Bain. As Tennant recalls: "They needed someone who could believably act 19 and bonkers." He could, and did.

After that he moved to London and rented rooms from Fast Show actress Arabella Weir, a landlady with the gentle yielding qualities of Davros, leader of the Daleks. Using the dishwasher was verboten, and she rarely allowed Tennant to switch on the central heating. "We used to live in near-freezing temperatures. Of course, the fact that I was from Scotland delighted her, because she thought: 'Oh well, you're used to it, you'll be fine.'"

By this point he was carving a career as a serious spear-carrier and then a star of RSC productions, yet he could barely contain his delight when, aged 30, he got a bit part in a Doctor Who adventure on audio CD. "I got to play a Nazi-ish character and Sylvester McCoy was the Doctor. Sylvester McCoy was a bit after my time, to be honest, but he's still a god among men because he's played the Doctor. I played it quite cool, though. I don't think they knew quite how overexcited I was."

By 34, he had been talent-spotted and targeted by the BBC, casting him in a series of high-profile heavyweight dramas: He Knew He Was Right, Blackpool and Casanova preceded Tennant's metamorphosis into the 10th Doctor.

Despite initial scepticism from some quarters over Tennant's youth, the series appears to have found a balance under his insouciant performance. The hyperactive storytelling is clearly aimed at a new generation of kids glued to the internet or PlayStation, with enough invasions, monsters, classic foes, references to earlier series and "let's-not-take-any-of-this-too-seriously" sophisticated playfulness to please diehard fans.

But there is also an emotional dimension now, a departure from the classic structure of Doctor Who of simple morality plays wrapped in the distracting kitsch of period science fiction. Tennant's Doctor not only could get chased by a Sontaran through what looks like a Tiger Bay leisure centre after hours, but was also given to exchanging Basset Hound looks with his comely assistant Rose across a sonic screwdriver, Still, Doctor Who may be the exception to the rule that the motor of all drama is sex. Tennant says that while the upcoming series may contain a kissing scene, the relationship with his new assistant remains essentially demure and platonic because "you can't have shagging in the Tardis".

Off-screen, Tennant's relationships have consisted of a string of long-term affairs, including four years with Shameless actress Anne-Marie Duff, now married to Tennant's friend James McAvoy. Tennant's current girlfriend is Sophia Myles, who he met when she starred as Madame De Pompadour in Tennant's first season as the Doctor. Sophia is the daughter of a vicar, while Tennant's father, the Very Rev Sandy McDonald, was a minister and then the Moderator of the General Assembly of the Church of Scotland in the late 1990s. As for Tennant's own Christian beliefs: "I'd say that's an ongoing question for me."

So far, Tennant has managed to stay wry and dry about his elevation to saviour of the galaxy: "When I got the role of Doctor Who I thought my life would never be the same again and that I would be recognised everywhere I went. But when I turned up at the BBC in London to do a morning show recently they didn't recognise me and wouldn't let me in. They thought I was some bloke called Stephen."

In career terms, Tennant clearly has ambitions beyond Gallifrey and would prefer to make the break before he gets eternally typecast as the Time Lord, a fate that Jon Pertwee and Tom Baker did not escape.

To that end, he has already been exploring other options. Last month he appeared to great acclaim in the one-off drama Recovery as a brain-damaged builder, and later this month he begins filming the romantic comedy Cheerful Weather For The Wedding, opposite The Devil Wears Prada actress Emily Blunt.

It seems unlikely that Tennant will stay long beyond the fourth series of Doctor Who. Tennant himself has declared he wants to leave the show on a high, even though he remains coy about naming the date.

"It's judging when is the right moment to go. I think I now know how many series I'm going to do - but I think it would be stupid of me to say more."

Source: Scotland On Sunday 25/03/2007