Can we believe the Scots?

Nicola Sturgeon's assertion that SNP MPs will withdraw from the Commons for votes which definitely only affect England sounds reasonable. However what would happen if such a vote threatened a Labour minority government, say for example, on an England only mansion tax or on education policy? In other words would the SNP be willing to bring down a Labour Government and make a majority Conservative Government a likelihood? One could argue that the SNP thinks a Labour Government is better for Scotland than one led by Conservatives so the SNP do actually have a mandate to prop up a shaky Labour administration even if that means voting on purely English affairs. Semantics will mean everything here, I.e. what precisely is an 'English-only'issue? Alex Salmond is right that the budget is critical. A government cannot govern without parliamentary approval for its budget. Here is where SNP leverage will be at its greatest. Will they use it positively, as promised, or destructively? It is a matter of trust. Just one other point. Sturgeon is SNP leader but will not be a Westminster MP. So can she tell SNP Westminster MPs what to do, how to vote? What if they don't want to obey her; after all they have a different mandate to her. Intriguing.