At the SNP manifesto launchIf the SNP manifesto looked familiar it would be because it was a Labour manifesto wrapped in a yellow cover.Long-standing "progressive" Labour policies, like the 50p tax rate, the mansion tax, and the minimum wage were panhandled as if they were rare Tyndrum gold.You know when the SNP starts quarrying on the left it is not out of conviction, but convenience. That is where the rich seam of anti-austerity votes are to be found.Any inconvenience, like the £7.6 billion albatross of full fiscal autonomy, was effectively kicked into the political long grassAn independence referendum was dropped before the groans of an Edinburgh Assembly Rooms audience had time to die away.Sturgeon said she wouldn't bet on one in the next five years. But when she insisted the general election was not about independence, a hearty laugh echoed among the cavernous rock walls chosen for the launch.Beneath the SNP leader's strong and simple message about standing up for Scotland are layers of complex geology, aimed at different audiences.A grab for Labour's mantle of social justice combines with a presumptuous vow to deliver it across the whole UK.That soothes Scottish ears but by reaching out to England Sturgeon does a sterling job for David Cameron in marginal seats where the Tories evoke fear of the SNP to prod voters back to the devil they know.To ensure a Tory government, the result that suits the SNP best, winning Scotland is not enough. Labour must be weakened in England too.But should Miliband emerge as a Prime Minister Sturgeon promised her MPs would be a constructive force, on their best behaviour in the Commons.That makes perfect sense. To persuade Scots to vote SNP next year they have to be shown it was worth voting nationalist this year.The odd maverick gesture aside, SNP MPs would be obligingly supportive of Labour for a year, to prove Scots need not vote Labour again.In time a "betrayal" could be found and the walls of the Westminster temple brought crashing down, with the role of Samson played by Alex Salmond (sadly absent wrestling lions yesterday).You wouldn't bet against as referendum then.