‘Would you like a glass of pink champagne?’ asks Alex Salmond at 3.30 p.m., sounding very much like a man settling down for the afternoon. It’s Monday and Scotland’s former first minister has cause to celebrate. He spent the previous day musing on television about the price he’d demand for the SNP supporting Ed Miliband in the Commons, and his thoughts dominate the front pages. There’s plenty of outrage at the idea of the SNP toying with England, and outrage is just what he wanted. So champagne it is.

He has found himself an unlikely star of the Tory election campaign; the party this week released a cartoon showing him playing a whistle while Miliband dances. ‘I hesitate to offer any advice to Ed Miliband,’ he says, ‘but next time he gets teased or attacked by the Tories for getting into Downing Street with the support of the nasty Nats, he should say: “The Prime Minister has just conceded the election. We now know that I’m going to Downing Street, it’s merely a question of how I’m getting there.”’ Tory attacks need not be scary if ‘handled properly’, he says — Miliband just needs better advice.

He expects to be offering this advice fairly soon, especially if Miliband needs the SNP to assemble a majority in the Commons. Salmond’s successor, Nicola Sturgeon, has been busy destroying the Scottish Labour party, holding on to almost all of the 45 per cent of Scots who voted ‘yes’ last September. Polls suggest the SNP will take 39 of Labour’s 41 Scottish seats, becoming the third largest party in Westminster. In a hung parliament, they could decide who governs Britain.

What happens then? ‘Nicola is the party leader and makes these decisions,’ says Salmond, but she has ruled out any deal with the Tories. ‘The SNP approach to a Cameron minority government will be to bring it down,’ he says cheerily. But what if Miliband rejects his price? Would the SNP refuse to support Labour — and put in the Tories by default? Is that the threat they’re making? And if not, do they have any bargaining power at all?

‘Absolutely,’ he says. ‘The alternative to doing a deal is not doing a deal’: let Labour form a minority government, then exploit their weakness. The SNP, he says, could seek to amend legislation on certain issues, even joining forces with Tories if need be. ‘I’ve given the example of starting fast rail from Glasgow and Edinburgh to Newcastle,’ he says. He thinks such a scheme could be forced upon Labour by a ‘majority of the house: not just the Conservative party, Liberal Democrats, Labour MPs from Newcastle. People like that.’

So this is how Salmond sees the next parliament working out: the SNP not as anyone’s coalition partners but as political guerillas, deciding when to pounce. The English would not mind too much, he says, and he cites polls showing Sturgeon as the least unpopular party leader in England. But, I say, those same figures show Milband being far more unpopular in Scotland than David Cameron. How can the SNP justify setting their face against the Westminster leader who polls best in Scotland?

‘Because Scots still prefer a Labour/SNP government, and by a fairly overwhelming majority,’ he says. It sounds as if his mind is made up: the SNP wants to spend the next five years amending a Labour government’s legislation. But, I ask him, does this mean they would always vote in a way that kept David Cameron out of No. 10? ‘Correct: we would lock him out of Downing Street.’ So even if the SNP has no deal with Labour, it would vote in a way that put Ed Miliband in power? ‘Correct.’

This is a new development and — for the Tories — a deeply unwelcome one. Until now, the SNP was talking about doing a deal with Labour in return for its support, and Tories hoped that deal might fail. But as Salmond has just admitted, the SNP wants to enstool Miliband as Prime Minister anyway. Its preference, he says, is to support Labour ‘vote by vote’. There are survival votes for any minority government, as he knows having led one in Edinburgh. I ask what they are. ‘The Budget, basically. Also, keeping your ministers — unless you choose not to keep them.’ So on these issues, the SNP would vote in a way that would keep the Conservatives out? ‘Correct. But the Labour position would be capable of sensible progressive amendment — with our progressive allies, of course, Plaid Cymru and the Greens.’

It’s clear, then. Salmond won’t force Labour to do anything because he has already decided to put Miliband in power if he can — then gang up on him with the Greens and other nationalists. If Salmond’s mission is to prove that Westminster doesn’t work, then Miliband is his man.