Just as every good folk story needs a hero, villain and a dame, so does a strong campaign. And so the spirit of Robin Hood is invoked today by a coalition of leading NGOs and trade unions to put banks at the centre of tackling poverty and injustice. But who will play the roles of Robin Hood, the Sheriff, Little John?

Calls for a Financial Transaction Tax - now dubbed a ‘Robin Hood Tax' - are not new, but have been mounting rapidly as an innovative solution to three great ills of our time: a socially deficient financial sector, intense pressure on domestic budgets, and the continued existence of grinding global poverty.

Though not radical, the proposal is potentially far-reaching - a small levy averaging just 0.05% on a range of global financial transactions that could raise up to £450 billion worldwide, while calming some of the more volatile elements of the financial system.

Half of this cash would be spend investing in people and public services at home, half to finance our commitments to end extreme poverty and tackle climate change abroad.

It's an idea whose time has come. The economic and political stars are perfectly aligned for a once-in-a-generation chance to make international finance work for us, to build an opportunity from crisis.

The old technical barriers to such a levy have crumbled. The arguments are now moral, economic and political - but all winnable.

The moral argument for skimming the profits of a fat sector to support those who've suffered most from its excesses is easy. And just as the billion people who go hungry every night have had to bear the costs of our economic failure, so they must share in the fruits of our recovery - and see our promises to them kept.

The economic argument is clear too. Articulated best by heavyweights such as Joseph Stiglitz, Paul Krugman and Jeffrey Sachs, these economists not only value the immense revenue raised by this levy, but also its role in stabilising the more ‘casino-like' elements of an industry whose gambling addiction almost took us all to the brink.

But as ever, it is the political domain that remains contested, and it is where we see our Robin Hood storyline most. One does not have to think taxation is theft - who to the left of Hayek or Tebbit would? - to see a few parallels in the characters.

The role of King John in our tale is clear (think Fred Goodwin in a tiara), while the people of Sherwood are found in job centres and shanty-towns the world over.

In Merkel and Pelosi we have our Maid Marians, and in Brown, Sarkozy and Barroso we have our merry men. All are limbering up in their stockings and Lincoln green suits, arrows at the ready to support reform. All require that further signal to fire.

But whither Robin Hood himself? As always, campaigners' eyes turn to Obama to lead the outlaws, albeit nervously wondering whether he may be casting himself as Sheriff, allowing his Treasury foot soldiers to storm the forest and block a global deal.

So more than ever we need Gordon Brown and Alistair Darling - our own Little John and Will Scarlet - to use every ounce of persuasion to ensure our friend in Washington does not end up on the wrong side of history. As the leaders of the only financial hub to rival Wall Street, their role is crucial. As their electorate, so is ours.

We need the Financial Transaction Tax because it is the right thing at the right time, and because it has enormous potential to save jobs and save lives. But if the Robin Hood parallel holds strong perhaps it could help save a political career too.

The last time some Americans took Robin Hood to box office success - think Kevin Costner and Morgan Freeman - they helped claim the crown for a returning King with an unambiguously Scottish accent. Of course, some analogies can be stretched too far - King Connery votes SNP - but it could redistribute a few votes too.

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Steve Cockburn is an anti-poverty campaigner

To support the campaign for a Robin Hood Tax visit www.robinhoodtax.org.uk