There are many French people living in London, but does it really have the sixth highest number of any city? French media have been debating the accuracy of this often-repeated claim.
In a speech last year, Boris Johnson, Mayor of London, recalled a meeting with France's former prime minister, Alain Juppe, who was mayor of Bordeaux at the time.
Juppe told him, Johnson said, that he had the honour of representing 239,517 people in Bordeaux, the ninth biggest city in France.
"I got the ball back very firmly over the net, folks," said Johnson, "because I said there were 250,000 French men and women in London and therefore I was the mayor of the sixth biggest French city on earth."
There is a political dimension to it, because it illustrates the idea that the UK's capital is booming, good for entrepreneurs and brimming with jobs.
Johnson says there are 250,000 French people in London. Other British estimates say 300,000-400,000. The figure that is quoted most often is 300,000 which is attributed to the French consulate in London.
When asked, the consulate said this figure was for the whole of the UK, although most of those French people would be in the capital.
This is at odds with the figure from the UK's Office for National Statistics (ONS). It carries out a household survey once a year and its most recent one says there are 123,000 French nationals in the whole of the UK and only 66,000 are in London.
But these ONS household surveys don't count everyone. For example students in halls of residence and people in care homes are left out.
The UK Census does count this, and the 2011 Census says there are 86,000 people in London who hold French passports.
So why are the numbers so different? The ONS is absolutely sure about its figure. But the French consulate says the real figure is much higher.
Xavier Chatel, press spokesman at the French consulate, concedes there is probably no "official" measure.
"This is Europe so people are free to come, free to go, there's no authoritarian obligation to register somewhere and therefore there is a certain degree of uncertainty about them."
But he says that using data from the UK's electoral commission, the Department of Work and Pensions, Census data and the consulate's own register of people they can come up with a reasonable estimate of 270,000 French people living in London.
The consulate defines London as the city plus "the south eastern quadrant of the UK including Kent, Oxfordshire and maybe Sussex too".
This is quite a generous description of the London area - it includes Oxford, a city in its own right about 60 miles away from London. Kent and Sussex meanwhile, stretch right down to the English Channel.
But assuming the ONS number and the French consulate number are the best estimates, how do they measure up against the population of French cities?
"Like for like comparisons are pretty difficult because cities in Britain have fairly wide administrative limits. Greater London is way bigger than Paris," says Eric Albert of French newspaper Le Monde, which has been examining the "sixth city" claim in recent days.
He suggests the closest comparable figures come from the French statistical agency Insee (National Institute of Statistics and Economic Studies), which has a table of "urban areas" that are larger than the usual measures of cities' administrative areas.
London's ranking, using the ONS figure and putting it on a list of these French urban areas, is so low that it doesn't appear in the top 60. Using the French consulate figure, of 270,000, London is only the 23rd biggest French city.
The figures themselves are only half the story - there's a wider political point made by some that France's taxes are too high, jobs too limited and opportunities for entrepreneurs are better elsewhere.
The unemployment rate is higher in France (9.8%) than in the UK (7.2%) but what about taxes?
"It's rubbish. Income tax is roughly the same," says Eric Albert, who argues that parents would probably be better off in France because the number of children you have determines how much income tax you pay.
He says the only exception may be for high earners but they would be more likely to go to Switzerland for a better deal.
But what about the numbers of people leaving France? In a recent article in the New York Times, it was claimed the number had risen 60% since 2000.
"When people talk about people leaving France they say there has been a big spike, which is true," says Liam Boogar who runs a blog about tech start-ups in France called Rude Baguette.
But he says this needs to be seen in context - and despite the increase, there are more British people living in other parts of Europe than French.
"The most recent EU figures put France at number five in Europe for percentage of citizens living outside of the country, with the UK first."
This, he believes, says less about a French exodus than it does about a Europe with a very mobile population.
Additional reporting by Charlotte McDonald