Exactly

The Scotland act does not devolve constitutional matter to Holyrood and as such it is for Westminster to decide if and when.

The SNP are being incredibly dishonest over this proposal. They know that they won't win right now and they want to tie the referndendum in with the anniversary of the battle of Bannockburn in an orgy of anti-English jingoism.

Plus the SNP can always indulge in some anti-Tory dogwhistle politics due to the mess the Tories made of Scottish issues when they were last in power.