The report, commissioned by the Equalities and Human Rights Commission (EHRC), follows a 2008 report by the Institute for Public Policy Research (IPPR) in 2008 which also said half the migrants had returned.
But the Polish Central Statistical Office produces its own estimates of Poles working abroad. According to their figures, the total rose consistently until 2008, when there was a slight fall - a fraction of the British estimates.
They find they can live comfortably here on a minimum wage.
Ania Heasley of Ania's Poland
Professor Iglicka cited real figures too - not estimates - for the numbers of the returning Poles who have registered at their local labour offices.
She said they would have to do this to transfer any benefits earned abroad, or to claim benefit in Poland. The figures for 2008 were just 22,000 for the whole country.
There are as yet no figures nor estimates for 2009, but Professor Iglicka said she would expect a large number of returnees to affect the labour market or unemployment figures - both of which have remained stable.
She personally would assume that around a million Polish migrants - workers, dependents, students - remain in Britain.
Many Poles work in professions like plumbing
Dr Pawel Kaczmarczyk of the Centre for Migration at Warsaw University helped the Polish government set up the "powroty" or return website, which was intended to help entice the migrants back.
But he too says that the great return has not taken place. He also considers the British estimates inaccurate.
"Definitely 50 percent did not come back," he said.
It was the IPPR which came up with a way of calculating the numbers of migrants who have left, using existing data and some additional research. The second report by the Migration Policy Institute built on their work.
The IPPR have defended their methods - saying that their technique is robust, and the best that could be done with the data available.
Half of the one million EU migrants who have arrived in UK since 2004 have left
665,000 nationals from the 10 newest EU member countries living in the UK in the last quarter of 2007
Every local authority in Britain has registered Polish workers
Source: Institute for Public Policy Research
Director of Strategic Communication at the IPPR Tim Finch stressed that all such work came with a "health warning" and wouldn't be 100% accurate. He suggested that the missing Poles might have gone to another European country.
It is notoriously difficult to track such migration. In Britain there is no comprehensive source of information about eastern European migrants.
In 2004 the government did try to keep count by getting them to sign up to the Workers' Registration Scheme. But many migrants - self employed building workers for instance - were not eligible, so the scheme underestimates the total.
The quarterly Labour Force Survey, produced by the Office of National Statistics, is a useful tool. But it surveys 60,000 households and many academics say it has a tendency to miss migrant workers as they will not live in conventional households but in dormitories or temporary accommodation.
Certainly some Poles are returning to their home country. Przemyslaw Szkarnulis, 28, came back to Warsaw late last year after seven years working as a general builder in London.
He'd fallen in love with a Polish girl, and wanted to live with her. He'd saved some money - they're looking for a flat.
But he doesn't have enough cash to buy a home outright, and he can't get a mortgage, because after five months of searching he doesn't yet have a job.
He said it was far more difficult to find a job in Poland. "In London, if you lose your job, you wait two or three weeks, you'll find another one," he says.
Polish shops are an increasingly familiar sight in the UK
And he has become used to what he considers an easier, more "normal" way of life in Britain. His friends in London are urging him to return. He may go back very soon.
Ania Heasley runs Ania's Poland, a recruitment and translation agency in South London.
Operating since 2004, she has thousands of Polish migrants on her database. She said she does not see a lot of people going back to Poland - most are staying.
Some families returned to Poland at the start of the crisis but have since travelled back to the UK. "They prefer to sit out the recession in a richer country than Poland," she said.
The Eastern European migrants are often characterised as young, childless and well educated. That suits the government: the Immigration Minister Phil Woolas has said that "migrants come to the UK for a short period of time, work, contribute to the economy, and then return home".
However, Ania said her clients often do have families, and most don't have a university degree. Many come from depressed areas of Poland, where unemployment is still high.
"They find they can live comfortably here on a minimum wage. Food is cheaper - relative to salaries - and they no longer have to choose between a new pair of children's shoes or a nice dinner as they used to do at home. They have a 'decent life' as they would put it," she says.
Katarzyna Frandsen, by contrast, is just the kind of migrant worker the British government like to cite.
She came to Britain to study, developed a career in British television, and after a period in Holland has decided to make her permanent home back in Poland.
She'd married, and had a baby, "and then it's another step in life. You want to be close to your roots, and you feel safe and you feel like home. Which you don't feel in any other country when you are not a native person."
Surveys suggest that many polish migrants in Britain would also like to come home. But for the moment the data indicates that relatively few are actually making the journey - certainly far fewer than the recent British studies suggest.