MOUTHS hung open. Complete disbelief. What the hell had gone wrong?

This was a disaster day for the advocates of emptiness. Those who had passionately stood for nothing much, were left holding exactly that. Their dreams of dynasties had turned into dust.

September 12 2015 was never meant to be like this.

As with Syriza in Greece or Bernie Sanders in the US, Jeremy Corbyn’s involvement in the Labour leadership contest was supposed to be a sideshow, not become the main event.

Everyone knew that, left to their own devices, Labour’s main leadership contenders would chase orthodoxy into irrelevance. Collectively they would bore the pants off party members, the press and the public, long before anyone got close to a vote.

The Labour Establishment knew that a soupcon was needed to spice up proceedings; something that moved the debate (but not the outcome) just a little to the left. They could see the need for a little cabaret before normal service (political orthodoxy) resumed centre stage.

To be fair, not even Jeremy’s most loyal supporters expected him to turn into Alexis Tsipras. But the outcome of Labour’s leadership ballot was clear: Corbyn had won.

Everything had rested on the unanticipated public response to a rule allowing anyone in the country to pay £3, register as a supporter and have a vote in the leadership race. No-one ever thought they would.

But, for less than the cost of a large cappuccino, this is what happened.

First there were tens of people, then hundreds, then thousands. All those lunatics who believed in saving the planet seemed to have decided that a £3 punt on Corbyn might just be worth a shot.

The Labour Establishment went ballistic. In what was denounced as a “cappuccino conspiracy,” party grandees were already demanding a fresh leadership contest ... and a change of electorate.

Some said the Pope was behind it all. Others blamed Naomi Klein. What was clear, however, was that Corbyn benefited from the absence of leadership clustered all around him.

As the Paris climate summit approached, none of the major nations looked like taking anything more than platitudes to the event. And Britain looked to be among the worst.

China may have stepped up to the plate with some real carbon reduction commitments, but Britain had lurched off into an orgasmic pursuit of new airport building. Local authorities may have begun to find the courage to say “No” to frackers, but the government had gone looking for new ways to remove the public right to say “No.”

And as Labour’s wannabes lined up to say little, there was Corbyn siding with the public, promising to put clean before dirty, signing up to an energy revolution that would put consuming less before producing more, and to making jobs, skills and sustainability the centrepiece of a new economics.

Whether out of hope, frustration or anger, tens of thousands of people agreed enough to pay their £3 and vote for him — and look where it ended up.

Labour was about to be led by an accidental hero, and one with an agenda that seriously threatened to turn it from a Tupperware party back into a political party.

Cappuccinistas across the country may have been ecstatic, but the barristas — scattered around various watering holes of the House of Commons — went into instant revolt.

Sod democracy. This wasn’t just the cosiness of debates about austerity or austerity-lite that was under threat. Corbyn would surely be a threat to the afterlife too — the Jack Straw rates of remuneration MPs might expect (outside Parliament) from the corporate interests Parliament had always helpfully protected.

The remnants of New Labour were in no doubt that Corbyn would become a one-man walking disaster area. No Trident missile system. No everlasting subsidy for nuclear power. An end to the fossil fuel free-for-all. Shrinking the corporate welfare state in favour of restoring the personal one. Swapping the defence and international development budgets around, to put peace-building before warmongering. A radical decentralisation of real powers to local authorities and local communities. Where would it all end?

Not everything in the outcome could be put down to Corbyn’s charisma.

Jeremy had just been Jeremy. He had spent a political lifetime chasing lost causes: peace rather than war, decent wages, full employment and inclusive welfare, social solidarity, the planet before profit.

In fact, Corbyn had stood for pretty much everything the Tory press deemed unelectable. Some deemed him so unelectable that they even formed a “Tories for Corbyn” cadre, just to add to the mischief. What they hadn’t banked on was that his very involvement in the contest stripped bare the main tenets of what the government (as well as the opposition) had been standing for. All around him, the arguments were crumbling.

It was clear that the world had learned nothing from the last financial crisis. Gamblers were back in charge. Private speculators required public austerity to pay for their gambling losses. As a result, debt and default were throwing currency markets into chaos. And the only (orthodox) answer on offer seemed to be to “give corporations more rights to screw the planet.”

None of the other Labour hopefuls wanted to turn the IMF into an international stability fund, or pledged to oppose the EU-US TTIP agreement that would give corporations greater rights than citizens. None of them would replace extortionate PFI schemes with public-public partnerships involving pension schemes and common ownership principles. For the rest of the field, all this was just too political.

The trouble was that, whether you were Labour, Tory, Lib Dem or whatever, none of these issues would go away. And Corbyn just kept saying so. The game was up. And in the bigger world beyond party cabals, the public already knew the emperor had no clothes.

So what happens now? Sitting there with his head in his hands, Corbyn didn’t really know.

Until now his political life had been lived (virtuously) among Leonard Cohen’s Beautiful Losers. Now Corbyn had to become the captain more than just the conscience. He knew he would need to “phone a friend.” The good news was that thousands and thousands had signed up to take the call.

Across international landscapes the same message is getting home. Morality and ethics are forcing their way back into the language of politics. Markets may squeal in disapproval, but public survival instincts now demand that markets work on ecological and sustainable terms, or not at all.

As Naomi Klein’s latest book proclaimed, “this changes everything.”

And if it does, maybe we can survive after all.

• Alan Simpson was Labour MP for Nottingham South from 1992 to 2010.