Sir Ian McKellen, one of our most beloved actors, will next be seen breathing life into one of literature's most enduring detectives, Sherlock Holmes.

It seems an appropriate match, but in a crowded market, filled with memorable reincarnations from Benedict Cumberbatch, Robert Downey Jr, Basil Rathbone and Jeremy Brett, how does McKellen's version - in new movie Mr. Holmes - differ?

"The take of this film is that he wasn't fiction, he was a real man, and he wasn't really like the Sherlock Holmes that Dr Watson portrays in the short stories and novels," explains the actor, who has just celebrated his 76th birthday.

"That was the intriguing part of it, that although Sherlock Holmes is a part many, many actors have successfully played and are still doing, this was, at least, not a script that any of them had done before. And part of the newness is Sherlock Holmes is very old."

Indeed, set in 1947, McKellen's Holmes is 93 years of age for much of the film, with flashbacks to 30 years previously, and living in relative anonymity in the Sussex countryside, with his housekeeper and her young son, Roger.

Irked with the misrepresentation of him in Watson's novels, Holmes diverts his attention to an unsolved case, and is frustrated when his memory falters and he can't remember key details relating to it.

"At my age, I'm inevitably interested in what it's like to be an old man, surviving your friends, trying to make new ones and trying to understand a sometimes alien world," says McKellen, who is stylishly dressed in a leather jacket and patterned scarf today.

"It's not a fantasy world that he lives in, but a very real world."

Although the actor, who was born in Burnley and brought up in Wigan, empathises with his character, memory loss isn't something that concerns him.

"I don't worry on my own behalf about decline, because it's not really happened yet," he says, turning his spectacles over in his hands as he speaks.

"But I do with friends and people my age and a bit older. I see what happens, and mortality's ever present, of course. It's no fun seeing an elderly relative decline and change."

Now in his seventh decade, McKellen's commanding performances in the Lord Of The Rings and The Hobbit movies are just as celebrated as his long stage career and rare but highly-praised comedic turns in Extras and - for a brief but brilliant spell - Coronation Street, as con artist Mel Hutchwright.

But growing up in the Forties, cinema visits were "occasional" and "there was no television" to watch, he recalls.

"I didn't go to the ABC Minors [cinema clubs] all the other kids went to on a Saturday morning," he says, smiling. "I didn't go and see films relentlessly, week in, week out, and collect a whole bank of memories of films that other people have now forgotten. I'm not one of those film buffs!"

However, after studying at Cambridge, McKellen, who also stars alongside Derek Jacobi and Frances de la Tour in ITV's Vicious, a comedy about an older gay couple who work as actors, honed his craft at regional theatres.

"When I started out, you couldn't act unless you were a member of the union, Equity," says the X-Men star, who describes himself as a "slogger".

"Before you could get your full card, you had to be a provisional [actor] and you had to do 44 weeks of work - not continuous work - but 44 weeks of work, before you appeared at the West End or in a film or on television.

"So for the first 44 weeks of your career, you were not doing television, you were learning how to act in front of an audience."

Although the rules have changed, McKellen is no stick-in-the-mud when it comes to commending the new generation, praising both his younger colleagues on Vicious and 12-year-old Mr. Holmes co-star Milo Parker (who plays Roger) for their talents.

"Milo was full of the esprit of a young person," he says. "He had no fear of the camera or of doing exactly what the director wanted when required."

Praise is also reserved for the recent vote to approve same sex marriage in the Republic Of Ireland.

A tireless campaigner for gay rights, does he think using the referendum is a useful way of approaching the subject?

"To be a purist, I don't think it's appropriate that civil rights should be a matter for a referendum, it should be automatic," he says, adding that, although it was a "very good result", there were still an "awful lot of people" who were opposed.

But he hopes Northern Ireland will follow suit.

"I think there will be pressure, I hope, on Northern Ireland, part of the United Kingdom, to join the human race, grow up, be nice to each other like we are in England, Scotland, Wales and the Republic.

"It's a bit unseemly that there's a part of the United Kingdom that hasn't got the point yet, and I think the Irish vote will clearly have influence on other countries, perhaps those which have a Catholic past. But you know, it's odd; Spain was an early country to have same gender marriage and that's a Catholic country."

As one of the founding members of Stonewall, the charity which campaigns for greater equality for lesbian, gay and bisexual people, McKellen will be embarking on a nationwide school tour in the autumn to help tackle homophobic bullying.

Clearly, the pupils he meets could teach their elders a thing or two.

"I find the youth of today to be considerably more mature than their elders, because no one's born prejudiced, that's learned behaviour," he explains.

"If a school decides to impose good behaviour and generosity towards each other, children are going to respond."

And if anything, interacting with the younger generation leaves McKellen feeling good about the world.

"I went to a school the other day where 41 different languages were spoken. 41," he says, pausing to let the number sink in. "There's no discrimination in that school because the differences are on display and are heard.

"Once a person recognises that differences are to be treasured and variety is the spice of life, the differences between people are not frightening, and bullying a minority becomes utterly inappropriate," McKellen adds.

"So yes, I am positive about the future."

:: Mr. Holmes is released in cinemas on Friday, June 19