A useful rule in politics and business is that to exceed expectations you first have to lower them. Gordon Brown, promoting his G20 gathering on April 2, seems to have forgotten this. Since November 15, when the same cast of characters met in Washington, the “London summit” has loomed large in his diary. Not only would it save the world but it would also be the springboard for Labour’s revival.

Now the race is on to salvage even a modestly positive outcome from next month’s one-day summit in a draughty exhibition hall in London’s Docklands. The presence of Barack Obama on his first big overseas visit (he has already popped across the border to Canada) will help. Other leaders, however, particularly Nicolas Sarkozy of France and Angela Merkel of Germany, are disinclined to be swept along by the Obama tide. They are more worried about putting in place regulations to stop irresponsible Anglo-Saxon bankers bringing the world to its knees than measures that will add further to government debt.

Mr Brown’s quest for a global new deal is looking problematical. Unhappy comparisons are being drawn with another summit in the capital, the London Economic Conference in June 1933, which failed to lift the world out of the Great Depression. Yes, as long as we do not expect a one-day summit to wave a magic wand over a sinking global economy, there are useful things that can be achieved and part of the way forward was sketched out at the meeting of G20 finance ministers this weekend in West Sussex, chaired by Alistair Darling.

The global economy is in its worst recession since 1945, with world trade and output heading for its first fall since the 1930s. The urgent task is to stem the slide that followed the near-meltdown of global banking last autumn.

Despite differences of emphasis between Britain and America on one side, and Germany and France on the other, there is no conflict between that and taking steps to prevent the banks wreaking havoc again.

Plenty is being done, including fiscal packages worth a combined $2 trillion, aggressive cuts in interest rates and comprehensive banking rescue packages. These moves are in the right direction and in some countries – by no means all – there is scope for going further.

More importantly, G20 leaders must present their actions not purely as national responses to national crises but as a joint effort to lift the world out of the mire. Confidence is in short supply; there is a better chance of reviving it if people and businesses believe governments are pulling together and that international bodies such as the International Monetary Fund have the resources to step in and rescue economies brought down by the crisis.

Confidence will also be boosted if people think bankers will not be allowed to drag us down again. At one level that means outlawing the kind of bonus culture that encouraged greed and irresponsible risk-taking. But it also means, as Lord Turner of the Financial Services Authority will argue this week, much smarter and more searching regulation. “Light touch” regulation merely lit the blue touchpaper on a bankers’ boom. Change is needed and it is needed internationally.

The other key task at the G20 is to act as a convincing bulwark against protectionism, something its equivalent 76 years ago lamentably failed to do. This does not mean agreeing a communiqué and then returning home and signing protectionist measures into law. The London summit should stand up for free trade and mean it. Otherwise this recession could indeed become another world depression.