One notable statistic about Russia is that the mean wealth of its 110m adults last year was $10,980 while the median was $870. In other words, if the country’s assets were equally divided, the man in the middle would possess more than $10,000 but, in practice, his net worth is less than a 10th of that sum. This is the result of 110 billionaires controlling 35pc of the wealth.

Another result of these figures from the Credit Suisse Research Institute (and a Swiss bank should know) is that much of the money ends up elsewhere, in more stable and bankable places.

As one City of London lawyer observed to me this week: “If [President Vladimir] Putin is occupying territories with minority Russian-speaking populations, what about Kensington and Chelsea?”

A third result is that it looks as if the west has a simple and painless retaliation against Russia’s invasion of Crimea - the financial equivalent of a drone strike instead of a war. It could impose asset freezes and visa bans on a few selected oligarchs (perhaps seizing Chelsea Football Club from Roman Abramovich, the minerals magnate). That, some say, should do the trick.

I would call this the Magnitsky illusion, after the US law imposing visa bans on Russian officials who were held responsible for the death of a Moscow lawyer who investigated a tax fraud. It sounds like a targeted, surgical strike with few costs or drawbacks but it is unlikely to work out that way.

London and its financial services industry have profited in the past 15 years from the cross-border spread of Russian assets under Mr Putin (just as German companies have gained from cross-border trade). The City has reached an arrangement with the Russian newcomers - they can join the local club if they obey the local rules.If Mr Putin’s Ukrainian incursion continues, the City should brace itself for more than Russian oligarchs being turned away from Harrow School and Harrods. Apart from not buying gas from Gazprom, the most effective means of inflicting pain on Russia would be financial, not personal: denying support for its banks and making Moscow take the strain instead.

Punishing individuals would be easier and involve less sacrifice, which is why the US and European governments are happier to band the idea about than to consider more serious sanctions. It also has popular appeal, especially in ‘Londongrad’, where the influx of wealth has inflated house prices across swaths of Kensington and Mayfair.

Do Russia’s billionaires deserve to be as rich as they are? Hardly anyone believes that, so they are a soft target. As Bill Browder, chief executive of Hermitage Capital Management, the fund behind the Magnitsky Act, argues: “We have leverage against Russia we didn’t have in the past because the Soviets didn’t have money in London.”

Furthermore, although the workings of Russian’s political elite are murky, Mr Putin relies upon the support of oligarchs of various factions.

“They are the Kremlin’s key constituency and, if his leadership became a liability, they would get rid of him in the way that shareholders get rid of a chief executive,” says Samuel Greene, director of King’s College London’s Russia Institute.

There is a twofold problem with targeting oligarchs.

First, although it sounds curious in this context, there is a matter of principle involved. London attracts foreign investors both because of its expertise and the consistent rule of law. “What legal basis would you have for seizing their assets? You would have immense difficulty framing any law and the courts would throw it out,” says one City adviser.

Second, the logical target for visa bans is those who decided to invade Crimea, notably Mr Putin himself and his intimate circle. That is very unlikely to happen because it would be too difficult diplomatically. It would be perverse instead to pick upon people who are at least one step removed from the action but happen to be within reach.

Action against institutions rather than individuals would not only be more defensible but also more effective, even if it demanded sacrifices that no European government wants to make.

A briefing paper accidentally exposed to sight by one UK official declared that the government should: “not support, for now, trade sanctions . . . or close London’s financial centre to Russians”.

The City - or the various competing banks and professional services firms that comprise it - is right to be jumpy. It has just tightened the listing standards for initial public offerings by oligarch-controlled companies, only to find that its business is at risk. “The City should have a conscience and say ‘we will do our bit’, but I don’t suppose it will, unless it is ordered to,” says one lawyer.

The most punitive financial sanction would not be to block listings by Russian supermarkets such as Lenta, which squeaked on to the London market just in time last week, but to target state-controlled Russian banks. They are fragile, with weak capital and $30bn of loans to Ukraine, and they rely heavily on interbank funding.

If sanctions were imposed, cutting off international funding, Russia’s central bank would have to provide liquidity at the same time as trying to prop up the rouble with its dwindling foreign exchange reserves. It would be a far tougher challenge to Mr Putin than denying visas to billionaires he knows.

It would also involve sacrifice, both for the City and European banks, and for those caught by Mr Putin’s inevitable, angry retaliation. But if it does not hurt, it probably will not work.