A flurry of meetings have taken place between embassy officials and the Foreign and Commonwealth Office to provide reassurances that the purpose is not to put them in the dock (though there will be some who think that is not an unreasonable place for them to be). The diplomacy is delicate.
Nazi gold is a subject which raises passionate emotions. The term has become shorthand for assets looted by the Nazis, most poignantly from Jews who they then killed in concentration camps. The Nazi gold affair has also embraced the question of the wealth which many Jewish families secreted away in bank accounts, notably in Switzerland, where an inflexible adherence to its secretive banking laws prevented the accounts being reclaimed by survivors or their descendants when the war was over.
Many camp survivors are now elderly, and sorely in need of the money which was wrongly taken from them. Lord (Greville) Janner, chairman of the Holocaust Educational Trust, a lobbying group, has spoken emotionally on this. It was he who first suggested a conference as a way of speeding what recompense may still be possible for those in desperate want of it.
But, for once, even his language has been moderated. Lord Janner, like the Foreign Office, is conscious that some countries could be scared away if it looks as though demands may be made of them. He wants maximum attendance for a meeting on which much is being pinned.
The stated aims of the three-day gathering are: to pool available knowledge on the historical facts on gold looted by the Nazis from countries and individuals; to examine steps taken up until now to reimburse countries and compensate individual victims; and to examine the case for further compensation of individuals.
It is point three that is causing the problem. Inside Switzerland, there have been strong mutterings that its efforts so far - launching an investigation into the role of Swiss banks and setting up a historical commission and two different funds for those who suffered - have won it little credit in the outside world. The foreign minister, Flavio Cotti, appears determined to show contrition, but some Swiss remain suspicious that the conference will provide another opportunity for critics to point a finger. Only after British reassurances that Switzerland will not be made a scapegoat has attendance been agreed.
Germany has noted the international criticism heaped upon Switzerland with concern. Senior German officials are understood to feel they have gone to great lengths already to make amends for their country's wartime activities and are worried that the conference will not acknowledge that. Russia, meanwhile, is simply adamant it will return no "spoils of war", whatever their origin. Earlier this year, its lower house of parliament, the Duma, overturned a seven-year-old agreement that spoils plundered by the Red Army at the end of the war should be returned to Germany. Millions of Russians suffered; what it seized in 1945 from the Germans was only reasonable recompense, they argued. That some of the goods seized from Germany were not Germany's in the first place is a point they wish to ignore.
Yet the conference could certainly benefit Switzerland and possibly Germany and Russia too. Although all three will undoubtedly come under the spotlight, the Jewish organisations who are attending are just as interested in countries whose role has not been publicly questioned hitherto.
It is believed that the banks of Liechtenstein, for instance, whose secrecy laws are no less inflexible than the Swiss, could shed light on the whereabouts of some assets if they chose to do so. Although the Vatican is sending a couple of priests, it is unwilling to bend rules that prevent the opening of its files for 100 years, even though those, too, might help in giving clues to what was going on during the war years.
Yet without willingness and openness, attendance means little. Getting 150 delegates to Lancaster House is an achievement of sorts, particularly in the comparatively short time- scale of six months, but it must not be an end in itself. The Jewish community, to whom this conference means so much, has every right to expect that action will follow. This is the most promising opportunity yet to discover what happened to gold - and possibly other assets, such as paintings - whose whereabouts are unknown.
That is the problem with delicate diplomacy. There is a limit to how delicate you can be when the whole point of the conference is to raise difficult questions to which some may not want to provide answers. There are many countries, such as Portugal, Sweden, Turkey and Spain, where looted gold was traded by the Nazis to buy imports during the war. There are others, such as Argentina and Brazil, which were known Nazi boltholes. It is inevitable that confronting their involvement will prove uncomfortable.
Everyone knows that really. The logic of getting everyone together is that the moral pressure for further action will be inescapable, which is, of course, why countries like Germany and Russia are dithering. But if ever an issue demanded the setting aside of self-interest, this must be it. All those who might be able to help must take part.
Robin Cook may not quite have realised the diplomatic minefield into which he was leading his officials when he agreed to host the conference, but it was undoubtedly an appropriate grand gesture from a Foreign Secretary espousing a new ethical foreign policy. The Government should not be shy of it and the delegates should do their utmost to settle the matter. Whatever assets remain must be identified and given back to the Jewish community while those who have suffered are still alive to benefit from them.
Some people have asked why Jews are pursuing their outstanding claims only now, more than half a century after the end of World War Two. The answer is simple: it had not seemed possible before. It does now. When Mr Cook opens the conference in three weeks' time, the opportunity must not be wasted.