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People who tried to escape from East Germany during the Cold War could be shot, jailed and tortured. But the government was so short of money that some ended up being secretly sold - to West Germany, the country most of them had been trying to reach in the first place.

"I found myself at a police station on my own. The counter seemed so high because I was only a little girl and I remember the policeman asking: 'Why are you not crying?' I think about his words now and ask myself: 'Yeah, why wasn't I crying?' I suppose I was in shock."

Daniela Walther recalls the night she was caught trying to flee East Berlin. It was 13 August 1961. She was five years old.

Two days earlier her father, Karl-Heinz Prietz who was a reporter at a teaching magazine, had come home with a tip-off that the authorities in the German Democratic Republic (GDR) were going to close the border between communist East Berlin and capitalist West Berlin.

"He knew they were going to build a wall," says Walther, referring to the Berlin Wall, which fell 25 years ago, on 9 November 1989.

Knowing it would be all but impossible to move to West Berlin after the barrier was erected, Walther's father convinced her mother to flee right away.

"She was reluctant to give up her teaching job - teaching was her raison d'etre - but she agreed," says Walther.

Image copyright Getty Images Image caption Construction of the Berlin Wall started in August 1961 - at first it was mostly made of wire

"My father told us where to go, where we would try to cross, and we waited for him in this allotment. We stayed there on the night of 11 August, sleeping in somebody's shed. I remember my mother agonising and telling me to be quiet. I felt afraid."

The following evening her father came and led them to what he thought was a weak spot in the border, which was already quite heavily patrolled. "He went ahead and called for my mother to follow, but she froze - she didn't have the courage. I remember standing next to her, listening to my dad calling."

And then the guards appeared. "They came out of the darkness and arrested my father. They took him away - I didn't see him for another eight years," she says.

Walther and her mother were also arrested and taken to a police station. Her mother was sentenced to nine months in prison for being an accomplice to the escape attempt and Walther was sent to live with her grandparents in the village of Stockhausen.

"Being the daughter of someone who tried to cross the border was worse than being the daughter of a murderer," she says.

"My grandparents said: 'If anyone asks, tell them you're the daughter of Lilo,' who was my aunt in West Germany."

Image copyright other Image caption Daniela Walther while she was living with her grandparents

Walther adjusted quickly to her new life. "I was actually very happy. My grandparents had lots of animals, including a dog, and because of collectivisation everything was open - there was no private land or fences - so I used to go off exploring."

Walther even taught herself to ski. "I found my father's skis, which were far too big for me, and I learned to ski in the orchard - which was probably quite dangerous," she says.

When her mother was released from prison the pair moved to Potsdam, where their relationship became fraught. "My mother was really quite unstable," recalls Walther. She distracted herself by enrolling in the army's horse acrobatics team, and performing in the breaks at equestrian events.

Meanwhile, East Germany's economy was in free fall. Many skilled workers and intellectuals had fled and the Soviet Union was stripping the country of its resources. By 1964 the fiscal situation had become so dire that the authorities developed a scheme to sell political prisoners to West Germany. They called it haeftlingsfreikauf.

The Berlin Wall

Image copyright AFP

Construction started on 13 August 1961 when barbed wired fences were put up overnight

It cut off West Berlin from surrounding East Germany and East Berlin

There were more than 300 watchtowers

136 people are known to have died trying to cross it - though victims' groups say the true figure was closer to 700

On 9 November 1989 the border was opened and the wall torn down

"Between 1964 and 1989 some 33,755 political prisoners and 250,000 of their relatives were sold to West Germany, for a sum totalling 3.5bn Deutschmarks," says historian and author, Andreas Apelt.

"Both sides had an interest in the business - the GDR because it needed Western currency and the West because it wanted to save people from the inhumane prisons of the GDR."

Prisoners were also traded for commodities such as coffee, copper and oil.

However, neither side wanted the public to find out - the GDR because it didn't want to appear weak and West Germany because it didn't want to be seen supporting the communist regime.

So the operation remained clandestine - people were traded in darkened nooks of the underground railway, the U-Bahn, or sent across the border in buses with revolving license plates. The number plates would switch at the checkpoints, so as not to arouse suspicion on the other side.

In 1968, a price for Walther's father, Karl-Heinz Prietz, was negotiated. "He'd been in prison for eight years. He was tortured - he didn't explain the methods, but they destroyed his health. I don't think he saw daylight for years," she says.

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While locked away, Prietz had spent hours writing down the bedtime stories he had once recounted to his daughter. "He invented stories about two bears called Bumsi and Plumsi, who had lots of adventures," she says. "In prison he continued to write these stories in old exercise books - there was a huge pile of them."

However, prisoners weren't allowed to leave East Germany with anything except their clothes, so he sent the books to his wife for safe keeping.

Once Prietz was settled in West Berlin, a deal was made for Walther and her mother to follow.

"I really didn't want to go - I wanted to stay in East Germany with my grandparents - but the deal was for wife and daughter," she says. "I think they paid 100,000 Deutschmarks for us."

When the time came to pack their bags, her mother said there was no space for her father's story books. "She didn't bring them with us. She said there wasn't enough room, even though we took all this other junk. I have never forgiven her," says Walther.

The two of them were taken across the border via Bahnhof Friedrichstrasse, a railway station on the frontier between East and West Berlin. It was 1969 and Daniela was 13 years old.

"My friend Gudrun came with us to say goodbye. I was very sad to leave her. She said she would come and visit when she was 60, because you were allowed to leave East Germany when you were 60."

Image copyright other Image caption Daniela Walther still has old photos of her friend Gudrun

After bidding farewell to Gudrun, Walther and her mother were interrogated by the secret services of the UK, France and the US, which controlled West Berlin. "They asked all these questions and I remember thinking: 'Sorry my life is so boring,'" says Walther.

"My father was waiting for us on the other side. I didn't recognise him, which was very painful for him. He was crying." Having been so young when they were separated, Walther felt more upset about losing her friend.

Life in West Berlin didn't work out for the reunited family. Walther's parents split up and she had difficulty adapting to the unfamiliar school system.

"When you're a child, school is the centre of your world and I hated it," she says. "I went from being top of the class in East Germany to being at the bottom. My language teacher told me I would never get to grips with English."

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Determined to prove her teacher wrong, Walther persuaded her father to pay for her to attend a language school in the UK. "I was the apple of his eye - he would have done anything for me," she says. So in 1972 Walther arrived in the UK and before long went on to study languages at Goldsmiths College in London. In her second year she met her husband, Bill, with whom she had two children. Framed pictures of Berlin adorn the walls of their south London home.

Walther, now 59 and a teacher like her mother, is glad she left East Germany. "It was for the best - otherwise I wouldn't have come to the UK, met Bill or moved to London, which is a city I love," she says. "But if I had stayed, then I would have made a good life over there. People were well looked after and I agreed with the principles of the state - I still do - just not all the spying and oppression."

Although she disapproves of the idea of selling prisoners, she understands why it happened. "It was pretty mercenary of the East Germans, but they were being bled dry by the Russians," she says. "For West Germany it was a humanitarian effort."

Walther's father died in 1996, followed by her mother in 2010. After that night at the border their relationship never recovered. "She just couldn't decide," says Walther. "And that's what ruined them."

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