I don’t know whether the Prime Minister entered Rotherham’s Magna centre through the main foyer ahead of his keynote speech there on Friday, but if he had done, he would have been wise to heed the words of the sign immediately outside the door.

“Welcome to the abyss,” read an advert for the adventure centre’s bungee jumping attraction, part of a revived former steelworks.

Whether someone working in events management at the venue was working as a double agent, we may never know. But by the end of his encounter at this year's Convention of the North, the Prime Minister may well have been grateful to be swallowed up by the ground.

His speech started well enough, greeted with muted but respectful applause.

Many of his devolution promises will be welcomed, albeit perhaps more musical to the ears of northern leaders in less sceptical times, times founded on a greater degree of trust.

Nevertheless Greater Manchester will be handed power over its rail network , the Prime Minister revealed, something both Andy Burnham and council leaders here have been desperate to see over the line.

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“We will be generous with capital spending,” insisted the Prime Minister, before adding: “But we won’t be able to afford everything everyone wants. So choices will have to be made. If you want more than they can afford, then they will have to raise the money themselves.”

Whether this means 'you can have it, but you can whack up fares yourselves to pay for it' remains to be seen.

In fairness, many senior figures here privately feel that Mr Johnson, suspected by some of believing in nothing but his own ambition, does in fact genuinely see the merits of devolution thanks to his time at City Hall in London, or at least up to a point.

But that won’t be how the speech is remembered. The first sign that all was not well came before he even got to the announcement.

As the PM expounded ‘transformational local leadership’, a voice piped up.

"Like our MPs, Boris?" came the heckle.

"Yes, indeed," said the Prime Minister.

"Maybe get back to Parliament. Yeah? Why are you not with them in Parliament sorting out the mess that you have created? Why don't you sort it out, Boris?"

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With no Speaker available to rule on the intervention, security bundled the heckler out via the fire door, which initially they couldn’t open. Reasonably worrying for all of us in the room, not just the PM.

Then came the press. Northern press, I should add, as Westminster journalists had barely travelled north for the speech, apparently because they weren't invited. But no matter: we can take care of things ourselves.

The first question, from a northern BBC reporter, asked how he could deliver any of these promises if he can’t get an election, or Brexit. It drew applause from the audience, a sign of things to come.

But the Rotherham Advertiser had really come armed. “Welcome to Rotherham, Prime Minister,” said reporter David Parker, ominously.

“A few months ago you said in a radio interview that lots of local forces were ‘spaffing money up the wall’ - is that something you believe - on investigations of historic CSE [child sexual exploitation]?”

Across the room, the implication quickly sank in.

Welcome to Rotherham, indeed.

The PM floundered. “I think actually...that’s not what I said...but...uh...what I certainly can say is all such investigations, certainly, er, here, are extremely important and...uh...but the point I was making...uh...was that...we do need to backing our police to be fighting crime.”

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Finally, the line came to him.

“And that’s why we’re investing in 20,000 new police out on the streets and putting more into policing.”

The exact quote the Rotherham Advertiser was - accurately, of course - citing comes from an LBC phone-in the PM gave in March , while still a backbencher.

“I think an awful lot of money, an awful lot of police time, now goes into these historic offences and all this malarkey and you know £60m, I saw, has been spaffed up the wall on some investigation into historic child abuse?” he had mused.

The Sheffield Star asked about arts funding, which the PM could have dodged, if he had thought quickly enough. But instead he pivoted to a line that he immediately doubted himself.

We have a £3.6bn towns fund, he noted, before suddenly realising - then acknowledging - halfway through the sentence that Sheffield hadn’t been given any money from the towns fund.

Laughter. Not after-dinner-speech laughter in Mayfair. This was at the Prime Minister, not with him.

I asked a bit more about that towns fund, which is being targeted predominantly at marginal seats the Tories need to win or hold. Was this an attempt to buy votes?

More laughter rippled through the audience, then scattered itself through the PM’s rambling denial . Laughter came during other questions, too, including when ITV asked about the PM’s description of David Cameron as a ‘girly swot’.

Mr Johnson’s final thought, that David Cameron had a legacy ‘to be proud of’, was met with stony silence - only for the laughter to resume in belly form when compere Steph McGovern concluded, as the PM was barely off the stage: “I’d just like to point out I am a girly swot. And I’m proud of it. Let’s see who’s in their job for longest.”

Afterwards, I wondered if that had all been as bizarre as it felt. Passive-aggressively muted receptions are one thing, but cheers when a Prime Minister is criticised, at a conference his own government has funded? Surely this was new territory.

No, this was not normal, confirmed one experienced northern politico I ran into afterwards. They had never seen a Prime Minister get such a bad reception, anywhere, in 25 years, even at their lowest points.

“Usually even if the people in the room don’t agree with you, they still show respect for the office,” they added.

Outside, on a walkabout in Doncaster, the PM appeared to get a more positive reception - even if the woman who accosted him about austerity made it onto the news bulletins, as the man in Morley had done the previous week.

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But more people seemed to seek out selfies with the PM than scream in his face.

His aides will hope that forms part of the dominant trend. For inside, the PM had read the room wrong, reacted to the room wrong, and then hurried out through the back door.

Government had fought northern leaders to take over this year’s Convention of the North and to ultimately hold it in Rotherham, this modern-day Tory target, amid a row over who got to be in control of the conference - an event ironically dreamt up and originally run by northern leaders themselves, entirely independently. (Sometimes the most trivial battles turn out to be the most revealing.)

Having got his way, the PM is throwing the dice. He hopes memories of pit closures do not run even deeper than the pits themselves; that his natural charisma trumps decades of decline; that a vote to Leave could now become a vote for the Tories.

But while he will never have expected the most rapturous welcome at the Magna centre, he will need one at the ballot box if he is to overturn lifetimes of Labour voting in areas such as this.

If not, it could point to a gap between Tory strategy and reality. The PM had better hope that’s not an abyss, because it’s too late to change course now.