The election campaign is becoming increasingly dominated by a small party whose raison d’être is to preach independence from membership of a union it claims is hindering national ambition. But the party is not Ukip, which had been expected to play a big role in this election. It is the Scottish National Party, which seems ever more likely to hold the balance of power after 7 May and is determined to use it ruthlessly to its own advantage and to the furtherance of its sole objective: the dissolution of the United Kingdom.