Patrick Harvie MSP is the co-convenor of the Scottish Green Party, and, perhaps more memorably, the “voice of the irresponsible left-led anti-family anti-Christian gay whales against the bomb coalition”, according to the Daily Mail.

“My mum was very proud of that, yes,” grins Harvie when I mention this description. “It was hysterical. More or less as soon as I got elected [to the Scottish Parliament] in 2003, I got involved in the issue of civil partnership – we felt very clearly, family law being devolved, that the Scottish Parliament should at least debate the issue... And I think that the Daily Mail fairly quickly decided, ‘ok, that’s the gay one, we’ll have a go at him every few months.’”

One of the reasons Harvie entered politics was the campaign to repeal Section 28, and he had worked as an LGBT youth worker before becoming a politician. He is visibly proud of Scotland, 15 years after devolution, for passing equal marriage legislation with the third biggest majority of any parliament in the world. “That was a real moment of unity,” he smiles.

In a year when Scottish-American actor John Barrowman’s “Glasgow kiss” in the opening ceremony defined Glasgow’s Commonwealth Games, a Scottish politician with concerns like Harvie’s has much to be optimistic about. However, Harvie has a whole other political struggle on his hands, which despite the odds, he seems really rather cheery about: Scottish independence.

The Greens are in favour of an independent Scotland, a stance which continues to surprise some people, being from a leftwing party perhaps thought to be less enthusiastic about borders than the mainstream parties are. Harvie himself admits that support for the break-up of the Union is “not universal” in the party.

The general idea is that such a change would sufficiently shake up Britain’s political and constitutional establishment, which the party believes is wedded far too closely to Westminster’s whims. “Given that our political framework hasn’t significantly changed in nearly a century, since women got the vote”, Green party leader Natalie Bennett explained recently, “this is an opportunity to push for a written constitution with rights for citizens enshrined within it.”

In a similar vein, the Greens also support an EU referendum – although they would campaign for an “In” vote were it to come about. Their reasoning here is similar; an opportunity to rethink Brussels’ influence and the vested interests operating within the EU.

Harvie, who began leading the Scottish Greens in 2008 (his co-convener is Maggie Chapman), is up-front about the fact that the Greens’ pro-independence stance comes from a need for “dramatic change in our politics, our economy and our society”.

He says: “For me, it’s got nothing to do with flags, or 300 years of history; it’s about the future. And I think that the best way, not only of changing Scotland, but actually challenging the nature of UK politics and the way that it works at present, throughout these islands, is Scottish independence. It doesn’t give a guarantee of a utopian future, but it offers up possibilities that are closed to us at the moment.”

Harvie’s refusal to play on patriotism is a markedly different approach from his partner-in-cause, Scotland’s First Minister and notorious Saltire-huckster Alex Salmond. And although they are both fighting for the same outcome in the upcoming referendum, Harvie is quite clear about steering his campaigning and politics away from the combative mainstream debate:

“I think one of the things I’m happiest about with the way we’ve [the Green party] conducted ourselves is that we’ve shown it’s possible to disagree about independence in a spirit of respect and friendship, and I think that’s the tone of debate that Scotland deserves.”

Harvie is clear that Salmond has “won a really impressive mandate”, and is keen to acknowledge his “right to put forward his own view on the monarchy, or on retaining a shared currency – I’d rather see an independent Scottish pound – on a whole host of issues.”

However, he also argues that the Scottish independence debate on the “Yes” side shouldn’t be all about Salmond.

“The referendum is on one question only: should Scotland be an independent country? It’s not a referendum on everything on his [Salmond's] white paper on independence. It doesn’t endorse every SNP policy. But I think the smart folk in the SNP also understand that the Yes cause needs to reach beyond the people who have only ever voted SNP, or who like Alex Salmond as First Minister.”

Harvie, who is smartly dressed in a floral tie and waistcoat, down in London to deliver a speech alongside Bennett in the evening, decries the way the debate has been playing out in the media.

“These kind of, slightly artificial, mano-a-mano, one-on-one TV debates, I think, fail to capture the breadth of either argument. Alistair Darling wasn’t able to capture the breadth of the various flavours of devolution that the three UK parties are putting forward... And Alex Salmond didn’t capture the breadth of the arguments, the possible visions for Scotland’s future. These sterile, one-on-one debates – middle-aged man shouting at middle-aged man – are not the kind of debate that is happening throughout the country, in every town hall, church hall, school, or doorstep.”

Harvie doesn’t accept that the devolution offers from the main three parties in Westminster are enough for Scotland. His argument mainly stems from the Greens’ anti-austerity stance. “We don’t have budget control over Scotland’s finances,” he says, “so when the UK government cuts funding on higher education, or housing, or any of the services we call devolved, our budget goes down as well. And we can’t do anything to really fundamentally get away from that austerity agenda.” He champions what he claims would see a “demonstrable shift away from the political centre, ie not just the southeast, but the City of London, and the vested interests that are at the heart of the finance capital model.”

One challenge for Harvie and the Greens in the build-up to 18 September, other than achieving their desired outcome in the referendum, is to have their distinctive arguments heard above the louder voices in the SNP. Harvie is emphatic about this, conceding, “I don’t think there’s any great secret that I’m not a personal fan of Alex Salmond. It’s really important to remind people that you don’t need to like Alex Salmond to vote yes. Because this isn’t about him. This is about the country.”