The Robin Hood Tax has come a long way since I appeared in a Richard Curtis short film to help launch our campaign in February 2010. We now have 115 UK organisations from Oxfam to the Methodist Church and from Barnardo's to the TUC backing us and sister campaigns across the world. We have the support of more than a thousand economists, including Nobel Prize winners. And we have a number of governments including France, Germany, Spain, Austria, Argentina, South Africa and the European Commission backing us.

In the UK the Archbishop of Canterbury this week signalled his support for a Robin Hood Tax, adding to the pressure on David Cameron. Opinion polls show that by a margin of two to one, the public agrees.

It has not been a steady upward curve. When I attended the G20 summit in Toronto last summer, it looked as if determined opposition from the Canadian hosts may force the proposal off the table all together. But a mixture of political leadership by France and the continuing need in this age of austerity for G20 members to find new sources of revenue means I arrived in Cannes yesterday evening in far better heart.

As David Cameron and his fellow G20 leaders meet over the next two days, they are under enormous pressure not just to take action to fix the immediate crisis in the eurozone but to learn the lessons of the past three or four years and ensure that finance works in the interests of society and not the other way around.

Mr Meryn King was right on the money when he said: "The price of the financial crisis is being borne by people who did absolutely nothing to cause it."

At their last summit in Seoul in 2010, G20 leaders acknowledged their responsibility to people forced into extreme poverty as the economic crisis spread around the world via falls in trade and investment. They quoted a World Bank estimate that 64 million more people now live on less than 75p per day, that's more people than live in the whole of the UK.

Yet an analysis of governments' budgets released by Oxfam on the eve of the summit found the UK's insistence that it will stick to its aid promises is the honourable exception rather than the norm. Far from increasing, global aid is expected to fall by a massive $9.5bn (£6bn) between 2010-12. $9.5bn is enough to provide primary school places for more than half the children who currently get no education.

The Occupy protesters outside St Paul's have been criticised for their lack of concrete ideas about what should change. Yet there is a very simple idea - a tiny tax on the financial transactions of banks, hedge funds and other financial institutions - that could go a long way towards making finance pay its fair share to society. Such a Robin Hood tax of just 0.05% could raise tens or even hundreds of billions worldwide if applied to share, currency, bond and derivative transactions.

The billion hungry people in the world (more than the total populations of the US and EU combined) should not have to wait for economic recovery to receive the help promised. Nor is it just people in Africa who cannot feed themselves, as I found out when I visited a food bank in Putney of all places earlier this year.

Given the UK's leadership on aid, it is all the more disappointing that the UK, rather than enthusiastically supporting Gates' recommendation, is instead expected to block progress. The PM knows the US is strongly opposed, yet claims financial transaction taxes must be global to work. Has he not noticed the UK's own unilateral Stamp Duty on share transactions that raises about £3bn every year?

Today, Bill Gates will present a report commissioned by President Sarkozy that will recommend G20 leaders adopt Robin Hood taxes as a way of fighting poverty and climate change. It is a real opportunity and one the G20 needs to grasp.

My hope is that a group of willing leaders will use Cannes as an opportunity to press ahead with Robin Hood taxes to fight poverty in their own countries and overseas. It would be great if David Cameron joined them. For one thing it would make me and possibly the St Paul's protesters a little less angry.