Director Amma Assante chose to use the place in Botswana in her film, but found a vagrant living there, burning a hole in the floor.

"We had the same house that Ruth and Seretse first came to make their marital home," Rosamund Pike, who portrays Ruth in the film, explains. "It was lived in by a homeless person who had burnt a hole in the floor and it was in a very sorry state.

"We re-housed that gentleman and restored the house. Now this house is there for their heritage and foundation.

"It's very evocative because those photographs that I first looked at depict Ruth in that house. It gives you goosebumps. We had all the supporting actors from the community. That moment when all the women sing to Ruth, which was an unscripted, unasked for moment; it was a spontaneous gift of song that arose. That's a culture bringing its roots to bear on a movie."

And the actress will always have a special memory of the house in Botswana: "I actually kept rocks and stones from the earth in Botswana," she smiles. "I've got a little box of them."

During filming she and co-star David Oyelowo got the chance to meet Ruth and Seretse's son, who is now Botswana's President, Ian Khama.

"That was a very amusing encounter," Pike adds. "There's a very fun picture of me and David on either side of him beaming like we sort of have a right to be parental over this man, who's older than us. We went for breakfast at the White House equivalent at Gaborone and we also met one of the President's twin boys."