Drat. I have now wasted time that I did not have! I have refreshed my memory of both MacCormick's case and the later Gibson's case (the case about "evident utility") and it seems to me that although a theoretical (but in my view incorrect) case might be made on the basis of them for denying the power of the Westminster parliament to repeal the entrenched provisions of the 1707 Act of Union, the idea of Scotland being lawfully entitled unilaterally to secede from the United Kingdom is contrary to the basic precept of the permanent union reflected in Article I of the Act.
Whether secession would be legal or not, would probably not bother the House of Commons. I seem to recall that the House has no right to bind future Parliaments, nor to give away British sovereignty. The current Labour Party seem unaware of this though!