Richard but no-one among the important players actually holds that line. There is no message from a UK government saying that Scotland or any other part of the UK would be forced to stay in the United Kingdom. The big story in the Scotland On Sunday today is the suggestion (which is churned out at every election) from the UK government that they would need to raise border controls. There have been many stories from unionists saying this or that would happen but no-one has even suggested that Scotland would not be allowed to leave the union. Constitutionally it is not questioned. The mechanics of how it would happen and Scotland's relationship with the EU etc are debated but those are different matters.