Party Leader interviews: Patrick Harvie

But in Scotland, where they came sixth overall behind Ukip in the 2010 election and lost their deposit in 19 of the 20 places they stood, no such breakthrough beckons.

In keeping with the reputation he gained during the referendum as a straight-talker, the co-convener of the Scottish Greens doesn't bother to pretend otherwise.

"We'll make our best effort," he says drily on a break from leafleting in Glasgow.

"Obviously, we're never going to say never, but we do know that first-past-the-post is a broken system. There's a genuine multi-party democracy in this country that's struggling to assert itself under the weight of an electoral system that tries to forces people to choose between two guys walking into Number 10."

However that doesn't mean this election is a waste of time for the Scottish Greens - far from it.

Since the referendum, the party has more than quadrupled in size to 8700 members, more than 1000 of them (like Harvie in his fresher year at Manchester Metropolitan) ex Labour members.

Those activists need to stay fighting fit, so Harvie is using the coming weeks as a campaign camp, training them for the real battle he has in mind, the next Holyrood election.

"We're going to have the capacity to reach people on the ground, door-to-door, street-by-street level that we've not had in the past.

"I think that's one of the reasons why good polling numbers have not always translated into Xs on pieces of paper come election day.

"We've got that now. We're going to have a campaign machine and the resources to run it."

Between now and election day, the Scottish Greens' 31 Westminster candidates, many of them also gunning for Holyrood, will go through their paces too.

While the electorate can sample key Green messages: a £10 minimum wage, public services in public hands, including renationalisation of the railways, and more power for local communities.

Looking ahead, Harvie reckons the party can win one MSP in each of the eight regional lists for Holyrood, and perhaps two in the Lothians.

If that happened, it would see the respected land campaigner Andy Wightman become a Green MSP. "He'd be a terrific addition," says Harvie.

With Labour appearing, as he puts it, "bewildered" and "fatalistic", and in no kind of shape for a 2016 win, a big haul of Green MSPs could give Harvie's party unprecedented clout.

"We now have a strategic opportunity in the Scottish political landscape," he says.

"I think even people who are fans of the SNP, a lot of them will be uneasy about the idea of swapping Labour hegemony for SNP hegemony, and they will want someone who can be constructive and challenging, and I think the Greens have an important role there."

It sounds like he's pitching his party as the economic and environmental conscience of a future SNP government, just as the SNP is currently touting itself as the conscience of a future Labour government at Westminster.

Would he consider coalition with the Nationalists if they fail to win a second majority and the numbers allow it?

"There's nothing objectively wrong with coalitions," he says, although he notes the Tory-LibDem one has been awful in practice.

"If you can do it honestly, if you can find genuine common ground, and put forward a programme for government that is not just in the interest of the country but consistent with what you were saying to voters for your reasons for standing, if that can be done, then it's conceivable."

The recent change of First Minister would certainly help such an alliance.

While he can hardly bear to discuss Alex Salmond - "My Mum always told me that if you can't find something positive to say you shouldn't say anything" - he calls Sturgeon "a very inspiring politician" who has already delivered positive change, such as the moratorium on fracking.

"I've never pretended to be a massive fan of Alex Salmond in terms of his policies or personal style, but that doesn't mean I can't work constructively on common ground.

"The fact that it's easier [with Sturgeon] on a personal level doesn't mean that it's necessarily easier at a policy level.

"If the SNP continue to move in a progressive direction then I will welcome that and I'll give them credit where it's due. But if they continue to want to shore up the fossil fuel industry then I'll continue to challenge that."

One obvious overlap with the SNP is a desire for independence.

A former member of the Yes Scotland board, Harvie has considerable experience of running a referendum campaign, and firm ideas on how not to run the next one.

For instance, he would also avoid another encyclopaedic White Paper, as it contained too many things for people to dislike, and gave the impression folk were voting for an SNP manifesto "rather than the first step of a journey".

Salmond's plan to share Sterling with the UK would also need to be junked.

"When the question arises again, that kind of half-way house is not going to cut it."

But before we rush ahead of ourselves, there's still the general election to get through, and a lot of conspicuously uncosted policies to sell voters.

Like replacing benefits with a universal citizens income of £100 a week for every working age adult and pensioner, and printing electronic money to invest in infrastructure.

Nor are there any projections in the Scottish Green manifesto for how much they would borrow, or raise through new taxes on wealth, land and financial transactions.

Surely they could have at least attempted to sketch out a balance sheet?

"I accept it as a criticism," he says without seeming too fussed by it.

"My instinct is to shy away from the retail politics. Choice in an election is not like the choice between mobile phone tariffs. This is a choice of ideas, about values, about the kind of society that we want to live in. That's what the Green party is always going to be more focused on."

But that's just a cop-out because you can't do the maths, isn't it?

It's better than making up numbers to put in a manifesto like other parties, he says.

"There's an obsession with people trying to kid each other they've got a crystal ball about the costs incurred by their policies and revenues raised from taxation. What we need is a direction of travel away from the low wage, low productivity, high inequality economy. It's not good for our values. it's not good for the decency and humanity of our society or the economy."

And it's true that on values he's far more impressive.

A righteous anger grips him when he talks about the Tory plan to cut £12bn from welfare.

He says the whole post-1945 welfare state in jeopardy because successive governments have simply stopped arguing for it, and there needs to be a fresh argument to save it.

"We've had decades of anti-welfare propaganda, a wave of argument from Labour and the Tories and some of the right wing media that says people are to blame for their own poverty and a welfare state should not be one that recognises everyone's equal worth and dignity," he says.

"Under the New Labour government there was the beginning of a shift away from a welfare state that's about people's wellbeing to a benefits system designed to bully people into low-paid work and subsidise low-paid work so that employers don't have to pay what people need to live with a bit of dignity in exchange for their Labour.

"That to me is not a morally or economically acceptable nature of a benefits system.

"We need to return to the principle that says everybody matters, everybody's well being matter, from cradle to grave we look after each other, that's what a civilised society does."

The Scottish Greens may not be about to sweep to power next month, but by playing a longer game, Harvie may well lift them to new heights next year.