Courtenay and Rampling hurdle the age barrier in 45 Years

A film starring two British "national treasures" set against a quintessentially English backdrop sounds like it might struggle to sell overseas. But 45 Years has been winning praise and awards - and raising eyebrows.

Charlotte Rampling and Sir Tom Courtenay take the lead roles in 45 Years as Kate and Geoff, a married couple looking forward to their 45th wedding anniversary.

But when a letter arrives about an old flame who died years earlier, it threatens to unravel their steady existence.

Despite the film's very English tone and setting, Rampling and Sir Tom shared a best acting award when 45 Years debuted at Berlin Film Festival earlier this year, and the film was snapped up by buyers from across the world.

But one of the biggest talking points has been the sex scene between Rampling, 69, and Sir Tom, 78. It only seems a shocking idea because the love lives of older people are so rarely portrayed on screen.

The scene was shot in 10 minutes.

"I loved it!" Sir Tom says.

"It was no bother whatsoever. It was so ungratuitous and it came so naturally and it was so true to life. I was much more worried about the dancing than the sex scene."

Rampling was not quite so keen. "I never liked them personally," she says.

"Tom loved it because he hasn't done many sex scenes in his career.

"I don't like it is because in that level of intimacy I feel very shy, but that's just me.

"Every other part of acting I'll give it my all and I'm not shy, but with this I feel you shouldn't have people watching, having to do it again or to be told where to put your hand... or bring your bum up a bit. I don't like that.

"But you do it with minimum people in the room - just three people. It's not a gang bang!"

There are no big fireworks - the film is more subtle than that.

The letter the couple receive reveals that the body of Geoff's former fiancee Katya has been discovered in the Swiss Alps, having befallen a terrible accident while they were mountain climbing together decades earlier.

Although this happened before he and Kate met, the news brings up haunting memories for him, and makes Kate wonder what could have happened if Katya had not died.

There are no action shots from the Alps or screaming rows - only reminisces at the kitchen table, photos uncovered in the attic and languishing shots of the Norfolk Broads.

The film was written and directed by Andrew Haigh, whose previous film Weekend won accolades and festival awards for its heartwarming depiction of a gay romance.

The cast and director believe they know why the film resonates internationally.

"I put it down to Andrew bringing the human out in people, and humans are universal," says Rampling.

"Race, religion and all that certainly go away when you have a story which is about relationships, and we are haunted by regrets. That's a very fundamental human condition to be in."

The actress adds: "It's not a film about old people. Hugely young audiences are coming to it and being impressed with it, and being moved by it."

Haigh himself realises that it could have been a hard film to market, but believes its themes have a universal appeal.

"For me, it is an English story and there is an English repression, but in the end it is still about the struggle that everyone has to understand not only their relationship but understand themselves and understand the choices we've made," he says.

"We've all had regrets looking back, wondering what could have been. That's such a universal thing. There's no person in the world at any age that hasn't thought 'what would have happened if I'd done that.'"

The screenplay is based on a short story by David Constantine, but changes the ages of the central characters from late 80s to the more sprightly 70s.

"I can't believe there's going to be any 15-year-old kids who usually see Transformers suddenly popping to their local cinema to see 45 Years," says Haigh. "But, you know, I'm 42 and it speaks to me.

"I haven't based the characters on anyone, it's more how would I react.

"There's this belief that you're fundamentally different when you're 70. I think that's based on our own understanding of our grandparents - but our grandparents as our grandparents, not as actual human beings.

"I don't think I'm fundamentally different than when I was 20 years ago. I have the same anxieties and fears and hopes so I feel that in another 20 years, I'll hopefully be a bit wiser, but I'll feel the same things.

"I love the observational way of film-making. I like the idea that people might reassess their parents' relationships, or their grandparents'."

Sir Tom is a veteran of British cinema, having made his name in "kitchen sink" classics such as The Loneliness of the Long Distance Runner and Billy Liar.

The promotional duties are getting to him. He confesses he has lost his hearing aid and jokes that "this is the third day of interviews, another tomorrow, and I'll be dead".

But the Hull-born actor is enthusiastic about this story. "I read it on my iPhone in an hour and I thought, this is it, the truth," he says. "A man looking back at what happened all that time ago in a very vivid way.

"It spoke to me in a way that doesn't happen much - almost never."

45 Years is released in the UK on Friday.