It is Glastonbury. Jeremy Corbyn stands on stage. A black circle opens up amid the grey beard as the absolute boy opens his absolute wet little mouth: “I have consistently opposed the European Union,” he thunders. “In 1975 I voted against EU entry. In 1993, I voted with John Redwood to oppose Maastricht. In 2008, I defied Gordon Brown to vote against the Lisbon Treaty.”

The crowd gawp.

“I was so lacklustre for Remain that I was largely credited with losing the referendum. And with my position – on us leaving the single market, on leaving the customs union, on erecting migration controls – I will make sure that we get the uncompromising exit I’ve committed to in the manifesto.”

Oh dear. The people came here to listen to a man talk about how every child should be allowed to draw a flower whichever way they like. They came here to rub the belly of the shiny talisman of Glastonbury’s venerable tradition of leftism. But now they’re coming into contact with Jeremy Corbyn’s actual views. Shit one… when are Chic on?

The throng of impressionable youth and even more impressionable media workers wilts. One by one they drift off, some in tears, some kicking the empty plastic cups as they go, each a deflated beachball of metropolitan socialism.

At night, around the tent circles, the chant goes up softly, a prayer, an offering, set to The White Stripes: “Oh Chukka Umunna.”

“Corbyn’s new strength, underpinned by all those young people who hate Brexit, means he has the political capital to crack down on his pro-Remain dissenters.”

In 2016, 75 percent of 19 to 24-year-olds voted Remain. In 2017, a 34 percent increase in youth turnout is widely credited with having swung Labour’s fortunes away from obliteration. And now that the debate is about to get unavoidably real, the difference between Labour’s position and the impression they put out there is about to become obvious.

It was always there if you read the manifesto closely:

“A strong emphasis on retaining the benefits of the Single Market and the Customs Union”

A neat sleight of hand. To cosy up to the pro-Brexit working classes, the phrase had temporarily stapled together these two constituencies. But like the chance to “win up to £50,0000″, it also includes the number zero: a “strong emphasis” depends on the deal. It’s an aspiration rather than a target.

Last week, that discrepancy began to unravel when 49 Labour MPs backed a rebel motion on the Queen’s Speech, proposed by backbencher Chuka Umunna, stating that the UK should stay in the Single Market.

Labour leadership tried to win back the rebels with a fudged counter-amendment: for a “jobs first” Brexit to deliver the “exact same benefits” as Single Market membership. No one was buying it. Fearful for their heavily Remain seats, four of Corbyn’s shadow ministers backed Umunna’s rebel amendment. They were sacked within hours.

Ironically, pre-election, they might have been allowed to linger in their jobs. Now, though, Corbyn’s new strength, underpinned by all those young people who hate Brexit, means he has the political capital to crack down on his pro-Remain dissenters.

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They’re not going to like his migration policy, either. Asked to rate their Brexit priorities, immigration came last among 18 to 34-year-olds, below “the need to share arts and culture between EU countries”, in 21st place.

Whenever shadow ministers are asked about EU migration, their first line of defence is usually to bluster about how they don’t want to see “unscrupulous bosses” hiring foreign workers directly from their country of origin to undercut British workers. As though it’s that easy: just ban the baddies. Yet, like Ed Miliband’s infamous “producer capitalists vs predator capitalists” distinction, when you break a policy like that down into case studies, you quickly realise that everyone’s a baddie but everyone’s also a goodie. So which do they actually want?

Again, anyone paying attention would know which side Corbyn’s on. Before the election he told ITV: “Clearly the free movement ends when we leave the European Union, but there will be managed migration and it will be fair.”

Free Movement Ends. Border Controls. No Single Market. Someone tell the meme team to put Corbs’ head on a rapper’s body saying that.

The Labour Party has been re-united in the wake of its stunning election loss-victory, but as Theresa May found out, honeymoons can be brutally short. Now, rhetoric meets reality. Now, Corbyn must choose.

Now, his 2017 youthquake might be in the same phase as the Lib Dems’ own youthquake in early 2010. Pyrrhic victory.

@gavhayes