William Hague is in no doubt about the most urgent issue to face him if he finds himself in the Foreign Office next month.

"It's the Iranian nuclear programme," he says. "We have consistently been the party arguing for tough sanctions and a strong European approach over the last few years, and are very frustrated that this hasn't emerged strongly enough."

The ex-Tory leader has argued for Britain to adopt the same sanctions regime against Iranian financial institutions as the United States and to seek European agreement on putting a stop to investments in Iran's vast oil and gas fields.

Sitting on the edge of the stage at the Tories' press centre at Millbank on the edge of the Thames, Mr Hague looks relaxed despite the poll surge for the Liberal Democrats. Indeed, he is very keen to talk about Mr Clegg's policy on Iran, which removes the possibility of military action. "Unlike the Liberal Democrats, we don't say you rule out forever, any military action. However, we are not calling for that," he says.

He recognises that intervention in Iran could have devastating consequences, but says that removing the possibility of a military strike leaves Britain diplomatically weak. "The way I usually put it is that Iran getting the nuclear bomb may be a calamity, although military action may be calamitous. This is why we need peaceful pressure. But to simply take all military efforts off the table is reducing the pressure on Iran."

I ask him why the British public should believe he and his colleagues will have credibility and clout on the international stage, considering how long the Tories have been out of power. I suggest he and David Cameron will have to work hard to convince people that they should be taken seriously as players in the Middle East peace process, for example.

He says he told David Cameron as soon as he became leader in 2005 that a deep understanding of the Middle East would be crucial to his claims to be taken seriously as a statesman.

"We have to be steeped in the Middle East, way back to historical matters. Because you can't understand it without the history. That's been one of the failings sometimes with the Western governments." Although there are no real policy differences between the Conservatives and Labour on Israel, he is convinced a new government could help give fresh impetus to the stalled peace process. "I don't think there has been a strong enough British engagement with it in recent times. I think it's been stalled without much British attempt to help kickstart it," he says.

Perhaps the ideological common ground between the Tories and Binyamin Netanyahu's Likud might be a help. "You can overstress the ideological common ground," he says quickly, adding that it is not possible to compare Israeli and British political parties.

However, he notes that he has known the Israeli Prime Minister since his own time as Tory leader, when Mr Netanyahu was last in power. He has also met his opposite number, Avigdor Lieberman, and the Kadima leader Tzipi Livni. "Yes, we would attempt to use those relationships. Yes, we are friends of Israel," he says. "We are concerned that if a two-state solution is not arrived at soon, then it will never be. That would not be in the long-term interests of Israel and that is why we want to see all parties involved being prepared to negotiate."

Mr Hague was critical of Israel's invasion of Lebanon in 2006, a position he stands by, arguing that Israel was damaged in the eyes of the international community as a result of its actions. On Operation Cast Lead he was more sympathetic.

"We wanted a ceasefire, but we stressed it needed to be a ceasefire on both sides. That Israel was under rocket attack and that it was very important for both sides to respect such a ceasefire."

However, he says the report by South African judge Richard Goldstone should not be dismissed. "Goldstone raised some important issues, which all concerned have to address. And of course democracies and free societies are held to high standards and should be."

When it comes to relations with nations in the region hostile to Israel, Mr Hague is keen to emphasise that he intends to keep the dialogue open and is happy to defend his visit to Damascus three years ago. "I think it's very important for us to try to detach Syria from Iran, although it may be a long and painstaking process," he says. "So I went to Damascus in June 2007 before any Foreign Office minister went, because I think the Syrians need to know they have got people they can talk to, over a long time. Hopefully, the same actual people, which matters to them."

And what of those controversial Tory allies in the European Parliament? The Labour Party has consistently tried to make an issue of the Conservative Party's relationship with the right-wing Polish, Czech, Latvian and Hungarian parties in the new European Conservatives and Reformists (ECR) grouping formed last year.

In last week's tv debate, Nick Clegg famously described them as "nutters". Mr Hague said he was not surprised that the Tories' political opponents had used the new alliance to attack David Cameron. However, he said the British people should be reassured that these were mainstream parties.

"I have been surprised by the historical ignorance of some of our opponents in Britain, although I think it's willful ignorance," he said. "I think they are trying to use this, rather than being as ignorant as they look."