The Scots don't need a referendum
The UK Government draws up on its LEGAL BASIS from Union with Scotland Act 1706 and or the Union with England Act 1707 Article 4 of the Act of Union states: "That all the subjects of the United Kingdom of Great Britain shall from and after the Union………have the same Rights Privileges and Advantages"
However, since devolution the people of England & Scotland, have had different rights, privileges and advantages.
The treaty of union is therefore null and void and England or Scotland can withdraw using the provision of Article 61 of the Vienna Convention on the Law of Treaties .
Article 61 Supervening impossibility of performance
A party may invoke the impossibility of performing a treaty as a ground for terminating or withdrawing from it if the impossibility results from the permanent disappearance or destruction of an object indispensable for the execution of the treaty. If the impossibility is temporary, it may be invoked only as a ground for suspending the operation of the treaty.