More incredible, though, in today's post-ideological, soft-hearted Britain, is how Rosa came to be in Moscow in the first place - and how she came to be left there.
Rosa Rust was born in 1925, the daughter of William Rust, a devoted British Communist, and his wife, Kathleen. She was named, of course, after the German communist revolutionary Rosa Luxemburg.
Rosa took her first steps across the table in a prison visiting-room, where Bill Rust was one of 12 prominent British Communists held for sedition at the time of the General Strike. He was, her mother told her, more interested in news of the class struggle than in Rosa's battle with gravity.
In 1928, Rust was summoned to Moscow to work for Comintern, the Communist International, and took with him his wife and three-year-old daughter. There he became a popular figure, not least for his propensity to denounce his backsliding British comrades.
"He had a lot of admirers," says Rosa, who still speaks with a pronounced Russian accent. "My mother was `just his wife'." The marriage was doomed, and with her mother also swept up in the expatriate revolutionary scene, Rosa was left to her own devices. She had spoken Russian from the beginning, having learned it during a fortnight in a Moscow isolation hospital.
Soon, she was wandering the streets, or trailing the city's gypsies. This was not acceptable behaviour for a child of the ruling elite. In 1930, Rust was sent back to London to edit the new Daily Worker. Before he left, he secured her a place in what she calls "a specialised children's home for foreigners".
The Politburo had decided to establish the special boarding school in Ivanovana Niskienz, a textile town outside Moscow, for the children of "fighters against Fascism" and Communist revolutionaries, many of whom were living hazardous underground lives in their own countries. Among them, Rosa remembers particularly Tito's son, Jakov. "He was a horrible boy," she recalls. "I hated him." His idea of fun was to take some food and barricade himself beneath the floorboards.
But the foreign Communists came increasingly under suspicion in Moscow. Rosa's mother had stayed on in the city, working on an English-language newspaper. She acquired a lover, but when in 1938 he was declared an enemy of the people and disappeared, she knew it was time to leave. She visited the school and told Rosa she would return for her in a year or so.
But the Hitler-Stalin pact of 1939 intervened, and then, in 1941, the German invasion of Russia. Rosa was still in the country, with both her parents in London, apparently confident that she was in good hands. This was far from the truth. Separated from school friends, Rosa found herself with displaced Germans, shipped by cattle truck to the far reaches of Kazakhstan. There she spent three years, first in a collective farm, then as a smith, and finally in a copper mine.
Malnourished and partly blind, she wrote to her old school to ask for help. Only then did officials in Moscow appreciate that the death of the daughter of a leading British Stalinist would not be good for Anglo-Soviet relations. Rosa was sent a pass, allowing her to leave what was effectively a slave labour camp, and make her own way back. The train journey to Moscow alone took 17 days. Finally, she arrived in Scotland by way of a Murmansk convoy, with the war still raging.
Bill Rust, meanwhile, had been faithfully praising Soviet society, apparently unaware of his daughter's sufferings. He had also remarried. According to Mark Burman, a radio producer who has come to know Rosa well through making a documentary about her, Rust had never told anyone in London about Rosa's existence. "I don't think he was properly divorced and the Communist Party was all very proper at that time," he says.
There was no happy reunion. Burman's documentary ends with father and mother coming to meet Rosa at King's Cross station, and failing to recognise her. After that, says Rosa, she met her father occasionally, but they did not speak of serious things, particularly not her experiences in the Soviet Union. The story of her time there, says Burman, "conflicted with everything he believed in".
Instead, she lived with her mother, learned English at Regent's Street Polytechnic, and was taught to tap-dance by her newly-found cousins. Later, she got a job as a telephonist and receptionist with Soviet Weekly, translating instructions from the Russian embassy to her British editor. Then she worked for Tass, the Soviet news agency, translating and typing out news reports from Moscow Radio, until it was closed down by the Foreign Office as the Cold War began in earnest.
In 1949 she had married George Thornton, a student of Polish. Her father had died the year before, so she was given away by Harry Pollitt, the leader of Britain's Communists during the Second World War. The singer Paul Robeson appears in the wedding photographs. Such people were her only family. She says now that all her life she has shunned "three things: politics, religion and nationalism".
Today Rosa is 73. Next year, she and George celebrate their golden wedding. They enjoy classical music and pottering around, sometimes with their four children and five grandchildren. With a background such as hers, her most revolutionary act may well have been to live an ordinary life.
"It always appealed to people that it was exciting and exotic," she says, of her early years. "It wasn't exciting and exotic at all. It was a question of survival. I am all for the peaceful life."
`Rosa Rust' is broadcast on BBC Radio 4 on Saturday at 2.30pm