Smiley, fluent and softly spoken, John McDonnell sometimes comes across as a bit cuddly. Yesterday Labour’s shadow chancellor was interviewed by Iain Dale at the Edinburgh festival. He said he’s looking forward to a boating trip on the Norfolk Broads. ‘My wife and I sail. But we sail badly. People get off the water when they see us coming.’ He felt he deserved a break after working with the Tories on a cross-party approach to the Withdrawal Agreement.

‘No one should have to sit opposite Michael Gove for six weeks. I did it for the country.’

Iain Dale quizzed him about Labour’s immediate threat: Boris.

‘The guy’s reckless. The guy’s unstable,’ said McDonnell. ‘I will move heaven and earth to stop a no-deal Brexit.’ He added, ‘I think we will beat him.’

Dale suggested that Boris’s sunny optimism is a characteristic shared by winners like Tony Blair and Ronald Reagan. McDonnell shook his head. Optimism is a defining quality of his party, he said. ‘The transformative optimism of the Labour manifesto in 2017 helped us to close a polling gap of somewhere between 17 or 20 points during the campaign.’

But he won’t underestimate the new prime minister. Ken Livingstone, the former London mayor whom Boris ousted from City Hall in 2008, told McDonnell. ‘He’s the best campaigner I ever faced.’

McDonnell summarised Boris’s character. ‘He’s intelligent, manipulative and ruthless.’

Which doesn’t quite square with ‘unstable and reckless’.

Dale brought up Len McCluskey’s prediction that Labour will win the next election but without an overall majority. Would he consider a coalition with the SNP? McDonnell knows how toxic a Labour-SNP pact would be to voters in English constituencies. He quashed the idea.

‘We would form a minority government. If other parties won’t support our policies we’ll go back to the country and those parties will have to explain their stance.’

That sounded like a nod and a wink to the SNP. Let’s work together in Westminster but without a formal deal.

Dale asked about McDonnell’s family background and he remembered his mother who sold biscuits at a high street chain-store. ‘She was the best economist I ever met,’ he said, referring to her management of the family finances. Her message to her son was ‘Be kind’.

‘But you haven’t always been kind,’ said Dale. He cited a disputed video-clip in which McDonnell appears to encourage the lynching of Tory MP, Esther McVey. McDonnell hedged a little, suggesting that the context had been misunderstood. Dale moved on to a notorious McDonnell quote.

‘They [the Tories] are social criminals and one day, I warn you, we will try them.’

McDonnell shrugged. ‘I was angry. It was after the first round of benefit cuts.’

Dale pressed him. ‘But “try them”? Under what law?’

‘I might want to invent it,’ said McDonnell.

That couldn’t be clearer. An incoming Labour government will use the courts to prosecute and perhaps imprison Tory MPs for passing bills that Labour had failed to defeat while they were in opposition. That’s McDonnell’s less cuddly side.

He said he’s not interested in succeeding Jeremy Corbyn as leader. And he casually dismissed the hopes of all male Labour MPs as well.

‘It has to be a woman.’

He floated a trio of hopefuls. ‘Emily Thornberry, Rebecca Long-Bailey and Dawn Butler – a spectacular range of talent. They’re not career politicians, for them it’s a vocation.’

No sign of Diane Abbott on his shortlist.

Then, back to Boris. The prime minister once vowed to ‘lie down in front of the bull-dozers’ to stop a third runway being built at Heathrow. McDonnell, who also represents a west London constituency, is equally opposed to airport expansion.

‘This will be the iconic battlefield of climate change,’ he said, promising to use the law to halt the new runway.

As for Boris’s promise to fling himself into the path of the bulldozers, McDonnell said it was a sham.

‘I don’t trust him. And it’s not just me. I have a constituent with a JCB who drives around trying to find him.’

Dale and McDonnell agreed on one key point. Quitting the EU by 31 October.

‘I can’t see the pathway,’ said McDonnell.

Dale nodded gravely.