The devolution legislation set up a process for an 'official' referendum, binding on London (and Edinburgh) government; but requiring authorisation by the UK government - which is unlikely to be forthcoming. Either way, I expect a referendum next year 2022 (or at least date fixed in '22, for '23) - either an 'official' one for independence; or an 'unofficial' one, indicative for a real indyref - and if that was passed, over to you, Boris, what are you going to do now? Use the Army to keep Scotland in the Union? Otherwise known as a mega crisis...
The devolution process does not have any set precedent or rulebook for referenda, so there's no such thing as an 'official' referendum. But legitimacy is the key word, and it is also worth noting that no referendum is legally binding. The only thing that makes it binding is the commitment from both sides to abide by the result unless proven otherwise by any laws, statutes, or subsequent court action.
Just to avoid confusion for anyone who isn't aware, but devolved powers are powers which fall under the Scottish Government's responsibility that are otherwise not highlighted as reserved to the UK Government. In other words, if it's not listed as reserved, it's devolved. Referenda in this regard is a grey area, as the Scottish Government has devolved responsibility over the organisation of the democratic process for Scottish domestic issues (ie Holyrood elections, council elections etc), whereas the UK Government as a structurally higher office to the Scottish Government is responsible for the Union as a whole including it's domestic and constitutional agenda.
So that is why the independence referendum issue is contentious, and it is why in 2012 the UK Government agreed to temporarily transfer/waver the necessary powers for avoidance of doubt to ensure the claim of control over the constitutional process for Scotland derives from Scotland at the time that it was requested. It ensured the referendum claim was set in stone and legitimised as there was no contention or debate to where the power to hold it was accountable to. The process of temporarily transferring reserved powers, known as a Section 30 order, was granted as it was an easy way to avoid the can being kicked down the road and getting lost in Holyrood and Westminster committees and potential court action - which would have weakened the pro-UK argument further - and avoided any scrutiny over who has the ultimate say over referendum in accordance with English law, Scots law (see Joanna Cherry parliament prorogation case), and the devolution settlement.
The question over what makes a referendum 'official' has never been tried and tested in court, and because the 2014 referendum was done on a Section 30 order, that has become the de-facto process for seeking a referendum - where both sides agree it should be held, rather than one side holding the correct power and legitimacy to hold one over the wishes of the other.
If and when the SNP get the landslide victory in this year's election, and request a referendum, a refusal from Westminster will lead to court action to prove who has the ultimate say. Having been involved in the debate over 'Plan B' within the party, there is no consensus for a 'wildcat referendum' that claims to be binding. A proposal not too dissimilar to it (by Chris McEleny which proposes that any SNP majority would be used as a mandate to commence independence negotiations) was defeated by a landslide.
I suspect the pro-independence argument will prevail both for the legalities (ie Scots Law being equal to English law, as well as the Edinburgh Agreement and Smith Commission recommendations) and the democratic argument, as well as the non legal binding of the UK Government's prime argument of the utterance of 'Once in a Generation' during the 2014 campaign.
A referendum by 2023 would be my guess. Any later than that would risk a Yes vote turning the 2025 election into a proxy reactionary debate in the same way the 2017 general election was for the UK's place in the European Union and it's participation in it's wider institutions (which admittedly would do the UK Government some favours to winning over soft reactionary Yes voters).
In any case it's a win-win for the Conservatives in the long term. They keep Scotland, they're saviours of the union who broke us free from the EU shackles. If Scotland votes Yes, then the the Conservatives are all but certain to remain in government for the next twenty years on sheer numbers alone from the constituency boundaries and electoral trends unless something monumental happens in voting behaviour.
Indeed, the point I was trying to make to Clansman a page or so back is that much of the rhetoric from the yes side, both on here and wider, is "everything is greener, the details will all get agreed because everyone will want to keep Scotland happy" in exactly the same way as the leave campaign did. From we can choose what type of EU membership we will want because we're not some improvised communist Eastern European state (really, have you looked out the door recently?) to the deal with rUK will be easy as both sides want the same thing.
My point was never to imply that everything will get agreed based on what Scotland wants, my point is that membership of the EU and/or it's structures is more holistic to prospective members than what a) the EU wishes to have if they had their own way b) models that are already available. So in this regard proposing joining the EU on favourable terms is based upon what now member states have been able to negotiate for themselves. The same can't be said for leaving the EU in the first place, where the UK as we all know has set the trend on how this is done and how successful any negotiation can be done. So in that way, I would argue against your point that it is not 'in exactly the same way as the leave campaign did'.
And no, this is not based on some assertion that this is more likely because Scotland is 'not some improvised communist Eastern European state'. My point in regards to Eastern countries is that the EU structures help them level up where there are economic and standard of living shortfalls more than what they are used to integrate western member states with more established economic backgrounds into one aligning system.
Hope that clears it up a bit! (Yikes, I've gotten carried away a wee bit there!)