Jacket off, slumped on the green office sofa, DC, as some of his aides call him, looks a picture of airbrushed relaxation.

Yet the polls are narrowing, there is talk of a wobble – panic, even – and the personal attacks on Gordon Brown seem to be intensifying; more Punch, less Judy.

After suggestions that, amid all the pre-election nerves, he has told his colleagues to “get a grip”, is it not the case that the Conservative leader himself needs to do so?

David Cameron throws back his head, laughs and declares: “Our election campaign is going extremely well. We’re putting the choice in front of the British people which is: five more years of Gordon Brown or change with the Conservatives.”

He denies being negative and says, on the issue of his bruising attack on the PM over the three Labour MPs facing trial: “I was making a very reasonable point, which is that it was quite wrong to dither over these people.

“The Prime Minister had dithered and once my speech was out there, they clearly decided to act (ie, remove the whip). You can see I was being helpfully constructive.”

He appears unfussed about the narrowing of the polls, and notes: “A 10-point lead is exactly the same lead we had last October, if you want to get all psephological with me.”

So, he is not losing it? “Do I look like it?” he laughs, before adding: “No.”

The Tory leader is preparing for a speech to the Scottish Conservative faithful in Perth on Friday, in which he will rally his tartan troops to go once more unto the electoral breach and try to revive the party’s much-reduced fortunes north of the Border.

They are targeting 11 seats across Scotland and have high hopes of “turning the borders blue”, but know that if they get three or four seats this will be a major triumph.

Mr Cameron is fully aware of the argument that the Nationalists are praying nightly for a Tory government in London so they can demonise it and hope, as a Conservative cuts agenda begins to bite, that voters will flock to the Braveheart cause. Yet the Conservative leader believes he can successfully counter this. “This is where they’re taking people for fools because everybody knows there’s a massive budget deficit, everyone knows the whole of the UK is in debt, everyone knows there’s a problem.

“If Alex Salmond thinks he can fool people by saying: ‘Oh, if only we were independent, we wouldn’t have any debts, we wouldn’t have any deficit, we wouldn’t have any problems’; people know that’s tosh.”

When it is suggested many Scots still believe a blue rosette is a sign of anti-Scottishness, Mr Cameron nods, but insists: “It will be easier to prove that’s not the case if we’re in government.”

Warming to his theme, he continues: “Why not turn the thing the other way round completely and recognise, yes, the Conservatives are still held back in Scotland.

“One of the things that holds us back is perceptions about the past, and a new Conservative government in Westminster that starts to get to grips with the deficit, that gets the economy growing, that backs our troops in Afghanistan, that governs the United Kingdom with respect, that is a different Conservative Party to the one that was in power in the 1980s and people can see that, and see that all of Salmond’s warnings were just politically motivated and taking people for fools, we will be in a different situation.”

Unlike the other parties, who have unequivocally backed the Calman Commission’s recommendation to give more tax powers to Holyrood, the Tories have said they will produce their own White Paper on the issue. Mr Cameron does not accept this suggests his party is out of kilter on Calman.

He acknowledges there is “a need for greater financial accountability” and Calman will be his starting point.

“But I don’t really accept the criticism from Labour that somehow we’re not taking this fast enough when they may have produced a White Paper but I have not noticed any legislation being introduced. So they’re involved in a political positioning exercise.”

However, when pressed to give a guarantee that a Tory government would give Holyrood more tax powers in the next parliament, Mr Cameron appears less certain.

“Well, if you can tell me when the election is going to be, how long the parliament’s going to last. I’ve given a pretty clear answer. You’ll get a White Paper. You’re going to get the plans all before the Holyrood elections.”

As a trusty aide suggests time is up, I ask if, like the Prime Minister, Mr Cameron can manage up to nine bananas a day.

“I find one a day is quite enough for me,” he replies.