David Cameron declared after the Scottish independence referendum in September that the question had been settled for a generation and perhaps for a lifetime. How wrong could he have been? Though Scotland voted to remain part of the union, the margin of victory for the No campaigners was comfortable rather than crushing. It has galvanised the nationalist cause.

Polls point to sweeping gains for the Scottish National party (SNP) at the general election in May. The unionist parties have not responded adequately to that prospect, and they need to start to do so. The SNP is not simply a part of the complex electoral arithmetic of a fragmented party system. It is a threat to the territorial integrity and policy coherence of the United Kingdom,