“Are you sure I can’t get you a warm coat? You look freezing. Honestly, it’s no trouble.”

Benedict Cumberbatch is ridiculously, unfailingly polite. He remembers names, recalls previous meetings, asks questions, shows an interest, gives every enquiry deep consideration and, yes, will even attend to the needs of an under-dressed journalist.

Despite filming in the frozen mud of Belgium, on Sir Tom Stoppard’s five-part BBC Two adaptation of Ford Madox Ford’s Parade’s End, he is looking dapper.

As we march across no man’s land, the rising star of stage, small screen and, increasingly, film takes large confident strides while your correspondent slips and slides in unsuitable shoes. He makes time for everyone we pass on set, and sparks up as soon he can. “I really fancy a cigarette, does anybody smoke? Do you smoke? You healthy buggers.”

Just minutes earlier, Cumberbatch was rolling in the mud to give his character, Christopher Tietjens, an authentic WWI-officer-in-the-trenches look. Cumberbatch, who became a fashion icon with his dashing overcoat as the BBC’s Sherlock Holmes, is keenly aware of his costume’s significance.

I wasn’t really born with a silver spoon in my mouth, or as part of the landed gentry

“This is where it all came from – the famous Burberry trench coat,” he grins. As well as sharpening up his style, Sherlock established the 36-year-old as one of the most recognisable British actors of his time, and catapulted him into the big league.

It also changed our perceptions of him. Previously, he was admired but never loved, known for playing clever outsiders – notably his Bafta-nominated turn as Stephen Hawking in 2004 – yet never considered a pin-up, much less a dead cert for global stardom. That is changing.

When Sherlock’s ‘death’ came in the final episode of the most recent run, fans flocked to the forums suggesting how he might have survived. More than 10,000 people entered a competition to win one of 150 tickets to a PBS screening and Q&A event in New York. Cumberbatch is no longer our best kept secret.

By the time we talk again, just before Parade’s End airs, his roles in The Hobbit and the new Star Trek sequel are in the can and he is currently working on another huge film, Twelve Years A Slave, starring alongside Michael Fassbender and Brad Pitt.

“I’m just prepping that – I’m growing my hair to a silly length,” he says. “They have already started, which I am always nervous about because I really have to hit the ground running, but I am really excited about working with the director, Steve McQueen.”

There is a mischievous bent to Cumberbatch, which surfaces when the conversation returns to Parade’s End (pictured below, with co-star Rebecca Hall) and he is asked about the inevitable parallels that will be drawn between the series and a better-known period piece anchored in the Edwardian age. “You have to give Parade’s End your full attention,” he says, glinting, “which is why we never, ever claim to be on a par with Downton Abbey. That is easy listening, and I mean that in a respectful sense. It is highly palatable and enjoyable soap opera.”

As if to prove this, he reaches for his iPad (“Here we go, here we go… one second, bear with me… bloody modern technology! Ask me another while I’m looking”), searching for his favourite passage from the script to read, before delivering a whole scene between Tietjens and his would-be lover. He is keen to draw parallels between the Edwardian era that is depicted and the present-day economic and social malaise.

“We are living under illusions that are similar to Edwardian delusions of empire and power and state, where ridiculous trade-offs in diplomacy are discussed over fine wines between the landed gentry who are deciding the fates of millions,” he says.

“My character, Tietjens, is all about duty and how duty is much more important than your own well-being. We don’t have that now. Cameron’s Big Society is just a terrible bit of window dressing for the fact that the government is withdrawing its support for the basic fabric of society, and the most needy, the most poor, the most at risk, in favour of bailing out the money system, which we know now is teetering on the brink of utter corruption. It is all happening before our very eyes.

“The irony of the First World War was that two deaths – that’s Archduke Ferdinand and the other one I always forget – resulted in millions of deaths. Now, through rogue trading and betting on unstable equities, the actions of a few have bankrupted millions in this world. Yet still they are getting away with it. The system hasn’t changed, they haven’t been imprisoned…”

He admits the quiet anger that sees the character of Tietjens – a horse-drawn man in a motor age – rage against the idiotic war machine has stayed with him.

“While I understand the anger of rioters last summer, I also know that a lot of them were jumping on the message in order to politicise their actions, which was basically about a smash and grab,” he says.

It’s a theme he returns to repeatedly, a frustration with the way we live now.

“We have just lost sight of things. We live in a world where it is just annulments and pre-nups and everything is immediately changeable, including your partner for life – the vows and words don’t mean anything any more.

“People aren’t as good as their words or handshakes. They have kind of slipped into it, just being about immediate gratification. Sorry, this isn’t to spit in the face of people who do live their lives by principles, there are shitloads of people that do.”

Cumberbatch knows that many will not want to take a lecture on social responsibility from a successful actor with his upbringing and Harrow education. But his heightened post-Sherlock profile means his voice now carries greater weight, and he intends to use it.

“I might get things wrong along the way, and I know I anger a lot of people because I talk from a very privileged standpoint, but I can’t rewind the clock, I can’t alter the circumstances of my birth,” says Cumberbatch, whose parents are actors Timothy Carlton and Wanda Ventham. “I wasn’t really born with a silver spoon in my mouth, or as part of the landed gentry. My parents just worked fucking hard to afford me a very expensive education. I was very aware of it and made full use of it, which is part of why I am doing all right now. But I’m not just what the label makes me look like, having been to a public school.”

To further confound the stereotype, he also fancies himself as an action man. He gets around London on a motorbike, has a fondness for scuba diving and loves filming stunts.

“I’m always cast as sort of slightly wan, ethereal, troubled intellectuals or physically ambivalent bad lovers,” he had told me, when we met just before series two of Sherlock. “I’m here to tell you I’m quite the opposite in real life. I’m a fucking fantastic lover! I’m a man of the world – and yes, I like to have a bit of adventure in my life.”

It’s not just acting adventures he has his heart set on. “I’m building a home at the moment, and it would be nice to fill that home with love and life and children,” says currently-single Cumberbatch, who was in a relationship with The Thick of It actress Olivia Poulet from his time at university in Manchester until 2010. “That has long been an ambition of mine. I think I have been waiting to do it since I was 12, really.”

But before a disorderly queue of admirers begins to form, Cumberbatch says he is in no hurry. He’s also incredibly busy. “Yes, so I have lots to look forward to – not that I’m not fully aware of how extraordinary my current run of work has been.”

Cumberbatch admits it is unlikely we will see him on the small screen quite as often in future. Fans should savour the upcoming third series of Sherlock, then.

I have just had the most extraordinary variety in the past couple of years

“I have peers doing some pretty interesting film work,” he says. “You know, whether it is Andrew Garfield, Tom Hiddleston, Michael Fassbender, James McAvoy or Tom Hardy – people I have worked with or who are very close friends and are doing wonderful stuff.

“Where I have been really lucky in comparison to all of them, and I think I can safely say this without inviting their jealousy or anger, is that I have just had the most extraordinary variety in the past couple of years.

“But for film you have to play a long game, and all of them committed to that quite a while back. I am just starting to do that, where you wait, you campaign for roles.

“There will be time out of work, but to be frank, I could do with a bit of a break, anyway.”

So, the big question: when can we start calling Benedict Cumberbatch a movie star?

“Maybe after Star Trek, then I’ll become one, I guess,” he smiles, after a moment to consider. “But probably now! I don’t know, I mean, start calling me it, why not? I can be Benedict Cumberbatch, movie star…”

Film

Benedict Cumberbatch: “I know I anger a lot of people”