Until recently, an earnest public debate in London on the state of the global economy might have attracted an audience of a couple of dozen: some enthusiasts with an ideological axe to grind and a few elderly citizens looking for somewhere to keep warm. I half expected a similar turnout when I volunteered to debate the subject last week with John Micklethwait, editor of the Economist, under the chairmanship of the BBC’s Evan Davis. But 800 people turned up at the Royal Geographical Society in Kensington, and paid an entry fee. Such events are growing and attracting big numbers. There is an extraordinary mobilisation taking place, the biggest since the Iraq War. But the mood is very different: anxious rather than angry; curious rather than committed; interested rather than ideological. In the event, there was not much debate. We agreed on the deadly threat of economic nationalism as a response to the crisis and that, for all the greed, follies and instability of capitalism, there is not a viable alternative system.

There has been an enormous response to the stories which appeared in the Guardian and the Sunday Times earlier this month, based on information passed to me by a whistleblower, who alleges extensive, systematic tax dodging by the leading banks, Barclays in particular. When the banks are queuing up at the Treasury rattling their begging bowls and looking for taxpayers’ cash or guarantees, it is simply inexcusable to be devoting huge human resources to outwitting the UK tax authorities. The head of Barclays’s tax division is reportedly on £40m a year and he “saves” for the bank (ie, avoids) vastly more in UK tax. Occasionally, critics ask: “What is wrong with dodging taxes, provided it doesn’t involve breaking the criminal law? We all seek to minimise our tax bills, don’t we?” Leaving aside the large grey area where legal avoidance and illegal evasion overlap, there is a basic ethical point: don’t businesses and individuals have a duty to the society whose domicile and public services they use?

And when they want British government money or guarantees, the problem is a “no-brainer”. The issue is not just for Britain. Last week I hosted a meeting in the Commons for the campaigning group Global Witness, which has just produced an impressive report documenting the ways in which some of the world’s leading banks are complicit in helping several of the nastiest dictators loot their economies and line their pockets using offshore tax havens. The G20 must get to grips with tax-havens abuses. But much can and should be done at UK level to make companies and rich individuals pay their dues.

In midweek I met a group of schoolboys from Hampton School in my constituency to deliver a petition to No 10. The pupils have compiled a remarkably eloquent collection of writings on the Rwandan genocide and the petition is to raise awareness of that horrific event in 1994. It would be too easy to let the memories fade. As we went in to Downing Street, delegates from a G20 preparatory meeting were coming out.

I couldn’t help but feel that the solidarity and humanity shown by the boys provided a sounder foundation for a future international order than the G20 communiqué being prepared in advance by the official “sherpas”.

Friday is set aside for constituency duties. I have to get my head around a set of alternative proposals for a derelict site on the Twickenham riverside, one of those neuralgic planning issues which, in this case, has led to paralysis for more than a quarter of a century. Then I was trying to help resolve a parking issue which has created bitterness between a world-beating, hi-tech small company and local residents whose cars are trapped in narrow streets when deliveries are made. And then, a very long weekly advice surgery with around 20, mainly complex, problems: asylum-seekers from Palestine, Kosovo and the Congo; a Child Support Agency case; a cowboy-builder problem; a man who thinks I am a free tax accountant; a disputed house benefit claim; a homeless man who lives in a garage; a couple driven mad by a wild young man next door, when he is not in prison; a neighbouring borough’s cemetery charges; an equity release “rip-off”; an elderly, disabled man fined for parking in a bus lane while collecting medication. Nothing – not performing at PMQs, not interviews with Jeremy Paxman – is quite as emotionally draining as the weekly surgery.

The weekend is a tour of mid-Wales: a 400-mile round trip in support of three of my parliamentary colleagues. Each has assembled large groups of activists and meetings with local business people who tell me about the fight against recession as seen from the front line. There are some positive stories, but the basic message is the same – from Brecon to Cardigan and Aberystwyth to Newtown: shops closing; bloody-minded banks; the burden of business rates; bring back the lost manufacturing. Somewhat punch-drunk after my sixth speech and Q&A, I recover thanks to a long walk in the spring sun with my wife Rachel among the daffodils in a park in Montgomeryshire; and get ready to face the motorway again.

Vince Cable is MP for Twickenham and the Liberal Democrat shadow chancellor