Alex Massie has a great piece in The Spectator on 'Why an SNP Surge at Westminster Could Mean the End of Britain'. I recommend you read it. I suspect he is fundamentally right about the obliviousness of English voters to the 'Rise of the 45' in Scotland. The referendum might have been won, but the Union is far from safe for as long as a majority of Scottish MPs are from the Scottish National Party. Moreover the collapse of Scottish Labour undercuts some of the structural advantages the First Past the Post electoral system gives the Labour Party, meaning that in the current electoral climate it is very difficult to see either Labour or the Conservatives getting an overall majority any time in the foreseeable future, unless something very fundamental changes.
Another important question, however, is whether Scotland's Conservatives are as tuned in to the dangers of the current political maelstrom as one would expect them to be. In short, Scottish Tories have it in their gift to deny Sturgeon and Salmond at least 10 seats, and there are perhaps 10 more held by unionist parties in which a combined unionist vote could deny the SNP a victory, as the chart below shows.
It may be difficult for Chief Secretary to the Treasury, Danny Alexander, to hang on to his seat, even with a fair wind of tactical voting behind him (or so Lord Ashcroft's polls seem to indicate). The other 10 Lib Dem seats are probably salvageable if beating the SNP becomes more important to Labour and Tory voters than traditional tribal loyalties. This is particularly true in seats with a strong residual Tory vote like Northeast Fife, Edinburgh West, Argyll & Bute. This also applies in a couple of seats where before the SNP surge the Conservatives were hopeful of making gains at their coalition partners' expense: West Aberdeenshire and Berwickshire, Roxburgh & Selkirk. Returning the current coalition (Lib Dem) MP at least holds out the hope of the current government being returned; a SNP Member of Parliament makes that possibility even more remote.
If sufficient Labour and Conservatives vote tactically in Gordon, Ross, Skye & Lochaber and the Liberal Democrats' other Scottish seats, those seats could also be saved, though selling it to a mixture of Labour and Conservative voters becomes a lot more difficult.
The bigger challenge comes in Labour-held seats, where on current numbers the SNP may be about to sweep the whole lot of them away: can Labour drum up enough support in "Only we can stop the SNP" fashion to stem the nationalist tide? Can sufficient Scots Tories in East Renfrewshire be persuaded to hold their nose and vote for Jim Murphy? Similarly in Ayr, Carrick & Cumnock, Dumfries & Galloway, Glasgow North West and Airdrie & Shotts (!).
Nobody will know the answer until the early hours of May 8th, but if the SNP sweeps the board in Scotland it will be evidence that the continued existential threat to the United Kingdom remains little appreciated by unionists north of the border, let alone to the south.