If Michael Sheen is stuck for a script, he could do worse than write his own.

A fine actor, he has delivered impassioned speeches in his time, but none came close to the address he delivered in Tredegar on St David's Day to celebrate Nye Bevan and the NHS. This time, Mr Sheen was not impersonating Tony Blair.

Instead, a drenched Welsh crowd were given a glimpse of Bevan for the modern world. Or rather, they were reminded of beliefs unaltered in 70 years, of the Bevan who wrote: "The collective principle asserts that no society can legitimately call itself civilised if a sick person is denied medical aid because of lack of means."

For his part, Mr Sheen was striking out at the timidity and dishonesty of politicians who fawn over "our NHS" while conniving in its dissolution. His venom towards Labour, in Wales of all places, was unmistakeable. "You must stand up for what you believe," he said. "But first of all - by God, believe in something."

For Labour, that can be tricky. The party that derided any suggestion of a threat to the NHS before Scotland's referendum now offers itself as the saviour of the service. The party that warns of the grave dangers to Bevan's achievement did more "outsourcing", while in office, than the Coalition has yet managed.

Andy Burnham, Shadow Health Secretary, campaigns as a man who has seen the light. The fact remains: the process that led to Hinchingbrooke hospital in Huntingdon becoming the first private (now failed) NHS "operating franchise" began when he was Secretary of State. Under the guise of competition and choice, in defiance of public opinion, Labour embraced the private sector.

There are Blairites who still can't see a problem with that. Most are holding their tongues for now, for the sake of the General Election campaign. Some have been busily attempting to rewrite history, not least in Scotland. In Tredegar, Mr Sheen gave them their answer.

What's so important about an NHS in public hands? The actor said there is a choice. "Do we want to be a society where each person is recognised? Where all are equal in worth and value? And where that value is not purely a monetary one? A society that is supportive, that is inclusive and compassionate?"

This was the NHS as reality and symbol. Many politicians grapple with the problems of the former and are utterly baffled by the latter. Why should anyone care who provides treatment? Because equality in health care is dear to people in these islands. Inequality and its bedfellow, profit, trouble them profoundly where the NHS is concerned.

Belatedly, some who once promoted privatisation are beginning to understand that a collective instinct also has an economic point. It extends beyond health care. It overturns the quack remedies of the right. Inequality is as bad for a society, for an economy, as it is for a health service. Those who don't regard the fact as news have influential voices within the International Monetary Fund (IMF) and the Organisation for Economic Co-operation and Development (OECD) on their side.

Yesterday, Nicola Sturgeon, the First Minister, gave a speech in Linlithgow to publicise the Scottish Government's economic plan. Its novelty - in British terms at least - was in the explicit statement that the drive for equality will be at the heart of the SNP's economic as well as social strategy. Morality is not the only motive. According to analyses from the IMF and the OECD, inequality is a barrier, a big one, to a country's growth. Nye Bevan's "collective principle" is a lot more useful.

The argument has been developing for a while, driven in part by the experiences of millions since the bankers' crash. The rich have become richer, to an extent not seen since the late Victorian age, while the rest have footed the bills and suffered lectures on the need for ever more tax cuts at the top, the pointlessness of redistribution, or the marvels of privatisation. Opinion from the IMF and OECD now says that, even in terms preferred by right-wingers, this is nonsense.

Last December, the OECD reported that the richest 10 per cent in countries in its area earn 9.5 times more than the poorest 10 per cent. As recently as the 1980s, the ratio was 7:1. In 16 of 21 OECD countries, the Gini coefficient (zero means all have the same income; one means a single person has all the income) has risen from 0.29 to 0.32. In this grisly game, Britain trails only Israel, the United States and Mexico.

The OECD analysis concluded that the phenomenon has "a negative and statistically significant impact on medium-term growth". Let it go unchecked and the cumulative average loss in growth over 25 years is 8.5 per cent. According to the organisation's estimate, rising inequality has knocked "nearly nine points" off growth in the United Kingdom. And things have almost certainly become worse since the figures were compiled.

The OECD, sticking to economics, does not recommend redistribution for its own sake. Tax and benefits changes will do the trick, it admits, but only if applied with care. Nevertheless, its analysis shows that the right is deeply mistaken: "redistribution per se does not" - the word is stressed - "lower economic growth". Crudely, income inequality undermines educational opportunities, hampering social mobility and skills. The economy suffers.

Last year, the IMF reached similar conclusions: higher inequality means lower growth; the cost of redistribution is outweighed easily by the benefits. Supporting work by the economist Joseph Stiglitz, a paper produced by the fund dismantled every excuse offered for the growing gulf in incomes. Even when growth is achieved, inequality renders it "volatile".

Out on the campaign trail, Labour is as keen as any to denounce this curse. Past actions are forgotten, whether those were the failed Hinchingbrooke experiment, the vote to accept George Osborne's benefits cap, or dithering over the so-called Transatlantic Trade and Investment Partnership. As a kind of valedictory gift, Gordon Brown has meanwhile bestowed his plan for Scotland's economy.

Set aside the transparent plea for national unity: Labour is desperate. What does the man who presided over the bankers' bonanza have in mind? New technology, city deals, "a School of Finance", a North Sea PPP scheme, and (like every party) housebuilding. Once again, this would be "a modern equivalent" of home rule. The oil industry initiative aside - too little and far too late - these are not dire ideas in themselves. But nor are they notably coherent.

Mr Brown identified "big issues that matter, such as ending poverty, unemployment, inequality and injustice in Scotland". Quite where a School of Finance fits in was not immediately apparent. Jim Murphy's decision to accept that university students should not be charged tuition fees might do something for equality, but it is not what Scottish Labour believed just a few short weeks ago.

You can see what Mr Sheen meant by his cry from the heart. You can understand, too, that talking about inequality is a lot easier than acting on inequality. Nevertheless, the issue has become paramount. It will not be banished with another slogan on "our NHS".