Current polls suggest a UK 'yes' vote as most likely but polls from as recently as 2013 and 2014 suggest the opposite – an English 'no' and a Scottish 'yes' to the EU. So it is too soon to call it for the 'yes' side. And with the EU referendum possibly less than a year away – perhaps even June 2016 – Scotland's government would need to move fast if the UK retreated from Europe, or it might find Scotland on the outside of the EU too.
A UK 'no' to Europe would be a big shock – for both the UK and EU. Yet the political pressure after a 'no' would be for the UK to exit the EU rapidly, and so the fundamental questions and details of the UK's future relations with the EU would need tackling quickly.
But, if the UK started rapid talks on exit with Brussels, what would Scotland do, and how fast could the Scottish government organise a fully fair and democratic second independence referendum?
The Cameron (or perhaps no longer Cameron but still Tory) government may well oppose a new referendum saying it is too soon after the last one or too complex while the UK is disengaging from the EU. The UK government might add that other areas, London for instance, voted 'yes' to the EU too (quite likely) but that they have to accept the result just as Scotland and all other parts of the UK do. And it could argue that until Scotland saw the shape of the UK's future relations with the EU, how could Scottish voters decide independence in Europe was better than staying in the UK.
This could rapidly become a major clash between the Scottish and UK governments. In the face of a Westminster refusal to allow another independence referendum surely Nicola Sturgeon would hold an indicative referendum anyway – one that Cameron would probably declare invalid creating a huge political storm. The EU would also then be in a very tricky position – and doubtless stand back until Scotland and the UK were in agreement.
But other scenarios are possible too. If the gap between an English 'no' to Europe and a Scottish 'yes' is large, and if Scottish political momentum and polls veer strongly towards independence at that point, then refusing a new referendum may be difficult for a Tory government already facing the backwash from getting a 'no' vote when it campaigned for a 'yes'. Surely, by then, Cameron would anyway have had to resign.
And would a new eurosceptic, 'little Englander' Prime Minister both say 'no' to a new independence referendum and attempt to ignore a strong 'yes' result in an indicative referendum Scotland had gone ahead with anyway? Exactly who that Prime Minister were might determine the choice between a stormy and possibly failed attempt to defend the Union or a velvet divorce.
An English 'no' and a Scottish 'yes' to Europe would certainly be a momentous and challenging political and constitutional moment. But if these uncertain political waters were navigated and Scotland gave a dual 'yes' to the EU and to independence, the outlook then would look calmer if still complex to handle.
In the face of a dual 'yes', both London and Brussels would probably accept that it is the rest of the UK, minus Scotland, that is discussing EU exit. And Scotland could then negotiate its new member state status in parallel both to the UK's EU exit talks, and parallel to Scotland-rest-of-UK talks on dissolving the Union.
If the EU refused to negotiate on this basis, the priority for Scotland would then be to speed up the talks leading to independence. Once the UK has separated into Scotland and the rest of the UK (rUK), then it would be clear that the EU needed to negotiate exit conditions with the rUK, and staying as an independent member state with Scotland.
Managing three parallel, intertwined sets of talks would certainly call for high-level diplomatic and political skills: what Scotland can negotiate with the EU, is going to depend on what sort of deal the rest of the UK negotiates with Brussels on its own future relations – and on the independence talks between London and Edinburgh.
But even so, since Scotland is already part of an EU member state – meeting all its laws bar a few opt-outs – the talks with Brussels could be swift. Despite the sabre-rattling during the independence referendum last year, with the EU threatening Scotland with being at the back of the queue for several years, some in Brussels were apparently considering putting Scotland in a 'holding pen', whereby it remained in the EU while the details of its membership were sorted and accession treaties ratified – a classic EU fudge in other words.
The biggest linked challenges for a soon-to-be independent Scotland across the three sets of talks (only two of which it would be in) would be on borders and on currency. Would Scotland share the pound – and get at least a Swedish-style 'soft' opt out from the euro – or would England refuse to share the currency? And if the latter would Scotland then choose to start off with its own currency, not join the euro – the latter surely not acceptable in anti-austerity Scotland after the treatment meted out to Greece?
Much more difficult still would be the question of borders, which would depend on what the rest of the UK negotiates with the EU. Might the rest of the UK negotiate a deal with Brussels that allows open borders across Scotland, the rest of UK and Ireland to remain? This would be hard since surely after a 'no' vote, the rest of the UK would not still abide by the EU's free movement of people rules but Scotland (and Ireland) would have to as an EU member state.
This would risk Scotland – and Ireland – having to put physical borders in place, as their borders with England and northern Ireland would be part of the EU's external border. It may be that the new rUK would agree to keep borders open as long as both Ireland and Scotland agreed to remain outside the EU's Schengen – i.e. passport free – zone, but this could not be guaranteed ahead of agreement in the talks.
The irony here is that independence would be much more straightforward for Scotland if the rest of the UK stayed in the EU so that free movement of people and borders are not a serious issue. Yet it is a UK 'no' to Europe that would be the most likely trigger for a rapid second independence referendum.
There are tricky and potentially very stormy times ahead. The SNP have already aired the idea of a second independence referendum in the face of an English 'no' to the EU. But this prospect will need much wider debate across the UK before the EU referendum vote, to ensure broad political acceptance of this idea. If not, major political battles are likely to break out the morning after a 'no' vote and it will be rough waters ahead.