Today one of the most memorable Parliaments of the last few decades will be dissolved.

Five years ago, even with a minority SNP administration in Scotland and a new coalition government at Westminster, the basic features, or perhaps more accurately assumptions, of UK politics remained familiar.

Today the terrain looks very different. In London, hung Parliaments are now expected to be the norm, while in Scotland the concept of independence, which still appeared far fetched (to both proponents and opponents) in 2010, has been normalised to a degree few anticipated.

Meeting in Glasgow at the weekend your average SNP delegate (and certainly the party's leadership) was acutely aware they were among the main beneficiaries of this altered political landscape. And whatever the continuing weaknesses of their independence vision, few could deny they've earned it.

On Saturday there were allusions to the no-confidence debate which felled James Callaghan's government on that day 36 years ago, the circumstances of which, like so much else in Scottish politics, has been mythologised, not least Labour's persistent claim - repeated a few days ago to no real effect - that in 1979 the SNP's 11 MPs had ushered in the Thatcher era by voting with Margaret Thatcher in the House of Commons.

That much is true, although it's an academic point: even had the SNP voted with Mr Callaghan, his tired (and by then minority) government would have been compelled to take its case to the country within a few months. Michael Foot, the then Leader of the House, described Mrs Thatcher leading "her troops into battle snugly concealed behind a Scottish nationalist shield" but, at most, the SNP merely helped bring forward the inevitable: a Tory election victory.

What followed is well known - indeed the events of the 1980s still shape so much of modern Scottish political discourse - but also instructive in light of the on-going constitutional theorising emanating from the modern SNP. On one level, Alex Salmond, Nicola Sturgeon et al make a compelling case, but on another they're guilty of forgetting their own arguments.

Throughout the 1980s and '90s, for example, the SNP claimed a Tory Prime Minister with only minority support in Scotland (for most of her premiership Mrs Thatcher had around 21 MPs and 30 per cent of the vote in Scotland) had "no mandate" to govern Scotland. Now in constitutional terms this was nonsense - the Conservatives enjoyed an overall majority in the House of Commons - but nevertheless the UK government was perceived by a large number of Scots to lack legitimacy.

In yesterday's Sunday Herald Angus Robertson, the SNP's Westminster leader, rehearsed familiar refrains, observing that in 1979, 1983 and 1987 Scotland had "rejected Mrs Thatcher" yet had "to live with the consequences of a Tory government", a "democratic deficit" that manifested itself again in 1992 and most recently in 2010.

What's striking, however, is how the SNP is defying its own historical logic. In the space of a single paragraph, for example, Mr Robertson goes from railing against Tory governments Scotland "didn't vote for" being "imposed on us by votes elsewhere in the UK", to advocating a "strong team of SNP MPs" to ensure David Cameron "is locked out of Downing Street".

That, he added, was "Scotland's democratic right", but doesn't explain why Mrs Thatcher didn't enjoy the same "democratic right" to choose Scotland's government in the 1980s, but three decades later the SNP does when it comes to the rest of the UK. I suppose it could be rationalised as historical payback for 18 years of Tory rule, but that isn't how it's being presented by the SNP.

As both the present and former First Ministers have made absolutely clear in recent weeks, even if Labour wins fewer seats than the Conservatives, then the SNP intends to make sure the Conservatives are "locked out" of Westminster. I've yet to hear anyone adequately explain how that can be justified from an English perspective. Mr Robertson merely says such a scenario "would benefit the whole UK", although that's not a view shared by the majority of voters in England.

Last week the pollster YouGov asked voters if it would be "a good thing" if the SNP held the balance of power in the event of a hung Parliament - only 15 per cent agreed, while 63 per cent considered it very much a bad thing. Conservative voters hated the idea the most (91 per cent), but even Labour voters opposed it in principle by 55-22 per cent.

Despite talk at the weekend of "friendship" and "solidarity", however, the SNP appears to consider English public opinion irrelevant when it comes to forming the next UK government, but arguably it'll be central to the perceived "legitimacy" of any deal between one of the largest parties and smaller groupings such as the SNP, DUP and so on. The logic of Angus Robertson's argument is that one long-standing "democratic deficit" in Scotland would be replaced by another, considerably larger, deficit in England.

Now if, and this remains a big if, Labour emerges as the largest party then a deal, however informal, with the SNP would I think be defensible if not wildly popular in England, but in the event that the Conservatives have more seats and the SNP sustains Labour in power anyway, it becomes much less so. Now I don't believe, as some apparently do, that the consequence would be revolutionary, but it would certainly create considerable resentment in England, much as there was in Scotland three decades ago. And even if such an administration survived, Labour would suffer a backlash.

Parliamentary systems of government are subtle things, relying not only upon constitutional theory to function successfully but also consent - tacit or otherwise - from those being governed. Although it's a bit weird that the SNP are suddenly championing a Westminster "system" they hitherto condemned as "broken", they are certainly correct in theory, but in practice their preferred scenario is highly problematic.

Take the 2007 Holyrood election as another example. Then, the SNP became the largest party by a single vote but was nevertheless considered to have "won" the election, while its subsequent formation of a minority administration enjoyed legitimacy beyond those who had voted SNP. Now at that point the three Unionist parties could have teamed up to form a majority government, but somehow I doubt the SNP - backdating its own logic - would have considered that either legitimate or democratic and nor, I suspect, would a lot of Scottish voters.

One final point: following elections in February 1974 and 2010 incumbent Prime Ministers had a stab at forming governments when both parties (Ted Heath's Conservatives and Gordon Brown's Labour Party) had fewer seats than their main opponents. On both occasions those attempts failed, not least because it looked illegitimate for the perceived "losers" to try and cling on to power. In other words, "legitimacy" matters much more than some appear prepared to admit, and that's especially true of the forthcoming general election.