In an interview on the Andrew Marr Show yesterday, David Cameron revealed that he was planning to bring forward proposals which he hoped would settle the "uncertainty" surrounding Scotland's constitutional future in a "fair, decisive and legal" way. This raised suspicions that the Prime Minister was about to call a pre-emptive, Westminster-led referendum on the break-up of Britain.

In the event, nothing so dramatic transpired. Instead, Cameron has made an offer to Alex Salmond: hold an independence poll within eighteen months on the basis of a straight-forward Yes/No question, and Westminster will grant formal legal status to whatever result it produces. Refuse, and any future referendum run by the Scottish Parliament will be nothing more than advisory -- a kind of glorified opinion poll.

There are a couple of reasons why this must have appeared to the Tory leader as a clever political manoeuvre. First of all, if Salmond were to accept, he would forfeit the power to set the timing and wording of the ballot, both of which will be crucial in determining the outcome of the vote. Secondly, it hands a degree of initiative back to the Unionist parties, which have so far struggled to contain the SNP's juggernaut momentum.

On closer inspection, however, Cameron's intervention represents a rather clumsy and unthinking lurch into a debate he obviously doesn't fully understand.

The SNP is under no illusions about where constitutional authority in the UK lies. The nationalists know full well that for any referendum on Scottish secession to be binding, it would have to be ratified by the Westminster Parliament which remains - despite devolution - ultimately sovereign under the terms of the British constitution. It follows, then, that Alex Salmond has never intended to hold anything other than a non-binding or advisory referendum. What matters to him is that he secures a clear democratic mandate from the Scottish people to pursue the further transfer of powers from London to Edinburgh or, if he's really lucky, the creation of an independent Scottish state. The First Minister reckons he is more likely to get one or other of these things if he delays a poll until his preferred date of 2014 or 2015, after the full effects of the coalition's cuts have begun to bite and Scottish resentment toward the Tories un-mandated austerity drive has hit fever pitch.

But if the SNP rejects Cameron's offer -- and on the basis of this press release, it already has -- will it not be exposed to accusations of obstructionism? Is Salmond not taking a huge political risk by denying Scots the opportunity to vote sooner rather than later on an issue of such critical importance? One might think so. Yet, the opposition parties at Holyrood have been putting forward arguments like this literally every week since the SNP won a parliamentary majority last May and the only discernable effect has been to push nationalist poll ratings up, not down. At the last count, the SNP registered 51 per cent support, while Alex Salmond himself remains phenomenally popular.

So, before Cameron congratulates himself for having achieved what he thinks is an important political victory, he should ask himself a couple of questions. How often have Westminster politicians gotten into a tussle with Alex Salmond recently and won? Moreover, how seriously have they underestimated the resilience of Scottish nationalism and its appeal to Scottish voters? The Prime Minister may soon be forced to realise he is in a fight he probably can't win, with an opponent he can't quite comprehend.