Green leader suffers 'mind blank' and is unable to offer answers to key policies

Ms Bennett repeatedly lapsed into silence, had coughing fits and complained of a "huge cold" as she was grilled by LBC Radio presenter Nick Ferrari.

Asked about proposals for building 500,000 social rental homes, Ms Bennett said the move would be funded by removing tax relief on mortgage interest for private landlords.

But challenged several times on how much cash that would bring in, she floundered, saying: "Erm ... well ... that's part of the whole costing."

When the host moved on to ask about the overall cost of building the homes, Ms Bennett again appeared at a loss.

"Right, well, that's, erm ... you've got a total cost ... erm ... that we're ... that will be spelt out in our manifesto," she said.

Mr Ferrari: "So you don't know?"

Ms Bennett: "No. Well ... er."

After a few seconds' more stalling, the party leader gave a figure of £2.7 billion for the total cost of building the homes.

But Mr Ferrari shot back: "Five hundred thousand homes, £2.7 billion - what are they made of, plywood?"

After a long pause, Ms Bennett said: "Um ... At a cost of £60k per home ...

"What we are talking about, what we want to see is the possibility of homes being built."

Suggesting £60,000 would not pay for "much more than a large conservatory", Mr Ferrari asked: "How are you going to pay for the land?"

Following another silent pause, the politician started coughing.

Mr Ferrari: "Are you all right?"

Ms Bennett: "Yeah. As you can probably hear I've got a huge cold."

Mr Ferrari: "I'm terribly sorry to hear that ... You don't actually know what this is going to cost, do you?"

The Green leader said the party had a "fully costed programme" that would be published before the election.

She then came up with a figure of "£6 billion a year", but seemed unclear what it related to.

"We're also looking at investing ... (silence) ... we've got the fully costed figures here," she stumbled.

Mr Ferrari: "You've said that on several occasions."

After some more coughing, Ms Bennett managed to say: "Basically we're talking about an overall saving of £4.5 billion."

The presenter said: "Do you think you might perhaps have genned up on this a little bit more Natalie Bennett?"

After coughing again, Ms Bennett replied: "We're talking about a whole range of ... what we're talking about is a whole range of issues ranging from the economy, from the NHS, homes, climate."

Mr Ferrari said: "Yes, I've only time for two unfortunately. I wish we had more time."

At a press conference officially launching the Greens' election campaign, reporters asked Ms Bennett whether she had let the party down with the interview.

Baroness Jones, who was chairing the event, tried to block the question, saying: "She is not going to answer that, okay."

But thanking the peer for her "kind attempt to protect me", Ms Bennett conceded: "It was absolutely excruciating in the studio. All I can say is occasionally one just has a mind blank, that happens.

"I've been presenting the Green Party's policies up and down the country. I've been delighted to do that. I'm delighted with the response they get and I'm delighted to have the backing of all 54,000 Green Party members."

During the launch event in central London, Ms Bennett insisted the general election would be the "biggest, boldest campaign ever" for her party and told reporters that 90% of voters in England and Wales would have a local Green candidate.

Of the 509 constituencies they will fight, around 12 are key target seats, she said.

The party leader set out six key themes for the campaign:

• Rebuilding the economy so everyone gets a fair share;
• Putting the public at the heart of the NHS;
• Ensuring everyone has a secure, affordable place to live;
• Taking action on climate change to protect our planet;
• Investing in a public transport system to be proud of;
• Ensuring every young person who wants to can access quality education.

Ms Bennett said people had been "abandoned" by the current political set-up and insisted the cash-for-access allegations against former foreign secretaries Jack Straw and Sir Malcolm Rifkind highlighted the problems.

"The scandals of the past few days are just a symptom of broader, deeper, longer malaise in British politics," she said.

Caroline Lucas, the party's only MP, said the party has ruled out entering a coalition in the event of a hung parliament in May but is "open to" supporting a Labour minority government.

She said: "The experience of the Lib Dems over the past few years demonstrates the dangers of losing your identity and integrity.

"We would be open to supporting a minority Labour government on a case-by-case basis.

"Working with parties like the SNP and Plaid Cymru ... we would form a progressive alliance which would put real pressure on a minority government."

Baroness Jones raised eyebrows at the event when she appeared irritated by a number of questioners and told one journalist, who asked about the cost of the party's policies: "Yes, I wish I hadn't picked you, thank you for asking that fascinating question."

Ms Bennett apologised to party members for her performance in the embarrassing radio interview.

She told BBC 2's Daily Politics: "I had a very bad interview on housing this morning. I'm very happy to confess that and I'm very sorry to the Green Party members who I didn't do a good job, any kind of job of presenting our policies on.

"That happens, I'm human. One can have a mental brain fade on these things."

She said colleagues had been supportive and insisted the costings of her party's housing policy were "now in my head".

She told Sky News: "They have been very supportive and sympathetic."