I WAS at the Scottish Green Party conference in Perth yesterday. I know what you’re thinking. Ah, Perth, glorious spot, a jewel in the hills, set amid the flaming red and gold splendour of autumn. In other words, any old rot except the Scottish Green bit of that sentence. But this weekend’s conference is a reminder of how vital the fourth party at Holyrood is right now.

It’s always been a source of bracing ideas, transforming fringe notions into received wisdom, as well as a party that tries to rise above tribalism. But at the moment it’s important for another reason too: it holds the key to a Brexit-related referendum on Scottish independence, or Brindyref, as it’s inevitably been dubbed.

On one level this is blindingly obvious. The SNP minority government has 63 MSPs in a parliament where 65 is a majority, and needs the support of the six Green MSPs to pass a referendum bill, assuming Westminster grants Holyrood the appropriate powers. But sometimes the blindingly obvious needs saying because no one else is saying it. Certainly not Nicola Sturgeon, who laid the ground for another referendum at the SNP conference without even a casual nod to the Green votes upon which it would depend.

There is danger for the First Minister in this kind of approach. Not because Green support is seriously in doubt. In theory, Green Party members could mandate their MSPs to reject a referendum bill if they took the hump. But in practice they are likely to let the six decide for themselves. Green co-convener Patrick Harvie has also set a very gung-ho tone, talking of the prospect of a referendum before “the window of opportunity” closes with Brexit in 2019.

No, the danger for Ms Sturgeon is one of attitude. The impression is already forming that the referendum is an overwhelmingly SNP affair. In part, this is down to opposition parties, who want to make a vote synonymous with the government, because it fits their narrative of the SNP neglecting day-to-day business in favour of their constitutional “obsession”.

But it also helped along by the SNP themselves. By their launch of a single-party “summer initiative” on independence, for instance. By their neglect of the wider Yes movement. Even their hostility to parts of it last week, pricing alternative voices out of their conference.

We’ve seen this before. In the last referendum, the Yes Scotland campaign was basically the SNP in a false moustache. Behind the flimsy disguise, it was stuffed with SNP personnel and ran on money from SNP donors. And when it went broke in late 2014, after failing to source funds elsewhere, it was the SNP which stepped in with a £825,000 bail-out.

There was also the White Paper, the currency union plan, the oil numbers. The Greens and Socialists bit their tongues while the SNP grabbed the wheel. The danger for the Yes side is that it happens again. Because an SNP referendum would be a referendum on the SNP.

No vote is an island. People’s motives as the pencil hovers over the ballot paper are many and complex and rarely pure. Just as the EU referendum was, for many, a chance to kick David Cameron, so the next independence referendum will in part be a verdict on the SNP. It might be nice to imagine high-minded citizens marching to the polls with only their deep reflections on independence and the Union to guide them. But dream on.

If there is a referendum in spring 2018 - the earliest possible date, based on the timetable that accompanied the draft bill issued last week - voters will also be reflecting on 11 years of SNP government, on the state of the NHS, on schools, on cuts, on tax on childcare. No party lasts that long without mountainous baggage and cheesing people off in myriad ways.

Appeals to history and destiny won’t help. No-one is going to believe SNP claims about a final chance to secure independence, a once-in-a-generation or once-in-a-lifetime opportunity. People won’t see the next referendum as the last one, merely the latest one. The exercise itself will be proof that the SNP will hold a referendum whenever it’s in with a shot.

None of this means another referendum is a forlorn hope for the Yes movement. The polls may not be at 60 per cent, but they are remarkably healthy. But a miss is as good as a mile.

The over-identification of a second referendum with the SNP would cause problems. The Socialists, RIC, Rise, the Greens and the rest of the Yes rainbow all have their place.