The official photograph of the new British government

I asked David Cameron for some advice last week. I live in Stirling, I told him, the seat once held by the great Michael Forsyth. Since the Tory wipeout in 1997, the local MP has been Labour’s excellent Anne McGuire, though the Conservatives still come a not too distant second.

However the SNP now holds the seat at Holyrood and at this general election, given the national trend, it is a certainty that Stirling will elect either a Labour or Nationalist MP. While I’d like to see Cameron carry on as PM for another term, I don’t want to end up with a separatist as my representative at Westminster. Therefore I’m voting Labour. My question to him, during an interview for Saturday’s Scottish Daily Mail, was this: am I doing the wrong thing?

He looked thoughtful, and for one wonderful moment I thought he was going to high-five me, praise my political acuity and tell me to crack on. Sadly, he went with the predictable line: ‘I’m telling Conservatives all over Scotland, vote for what you value, vote for the team you support and that team is the Conservative Party. Look, I’m not responsible for Labour’s collapse in Scotland. I’m responsible for trying to engineer the Conservative recovery and the Conservative alternative. You’ll only get from me support for Conservative candidates.’

I have to say I wasn’t massively convinced he meant it. That final sentence seemed to have a silent ‘in public’ hanging at the end. And sure enough, when I recounted the chat to a Tory pal later, I got a different answer: ‘If I was living in Stirling I’d be with you.’ What the Scottish Conservatives need to do, said my friend, is be the pro-UK choice in those seats where it’s between them and the Nationalists. Stirling isn’t one of them, this time.

I’ve never before voted tactically, and I feel uneasy at the prospect of doing so. I like to choose on the basis of my national preference, and Cameron’s Conservatives come closer to my socially and economically liberal beliefs than any of the other options — certainly closer than Ed Miliband’s new socialism, a home-brew not yet fit to leave the bucket.

But the United Kingdom matters a great deal to me. The Nationalists have refused to accept that the result of last September’s referendum should put an end to the constitutional ding-dong for a decent interval. The awful Alex Salmond is still careering around the country like a drunk on a dancefloor, and if he wins in Gordon will lead the SNP army that Scottish voters are about to send south.

I’m also uncomfortable with what’s starting to feel like a one-party state in Scotland. The polls say the Nats will practically wipe out Labour in May — indeed, they have said this, day in, day out, for months. Last week, I saw one that suggested the party could win as many as 53 of our 59 seats. This, added to the overall majority at Holyrood, despite a voting system designed explicitly to stop it happening, is unhealthy and plain weird: it’s not as if they’re actually much cop.

There’s more. When Nicola Sturgeon — whose talents I respect — says that the Nationalists would use their new Westminster heft honourably and not seek merely to play games designed to bring the UK closer to collapse, I struggle to believe her. I look at her rubbishing of the Smith Commission’s report on the day of its release and feel she behaved dishonourably then. The fact that in the six-odd months since the referendum the SNP’s campaign for independence has barely eased is an affront to democracy. And jemmying Salmond back into Westminster, where he will be in a position of great influence, surely makes a mockery of any claim to constructive engagement.

These are perilous times for the Union. Cameron told me he thinks the new fiscal powers that will accrue to Holyrood as a result of the Smith process will bring greater stability. Devolution had previously ‘created parliaments and assemblies that had a lot of politics of grievance because you were responsible for spending money but not raising it, and you could always blame Westminster. I think this big element of tax devolution, while within a fiscal framework, could find a better resting place for devolution and I hope so.’

I hope so too — in fact, I think it likely. But there’s a lot of damage that could be done before then, particularly with so much of the Scottish electorate in a hyper-emotional and belligerent mood. The SNP is bound to use this febrile period to further weaken the bonds of union.

In a perceptive article at the weekend, the journalist and former MP Matthew Parris wrote about what he called Sturgeon’s ‘politics of sadism’. ‘Her plan is as simple as it is deadly: to run a minority Labour government ragged, kick it around, wreck its authority — but refuse to let it die. The consequences for the Union would be demeaning. The torture would continue until Sturgeon and Salmond judged the moment opportune, and the Scottish people (the only electorate they need to bother about) sufficiently softened-up for a second referendum on independence. This they would win.’

For all Salmond’s deluded huffing and puffing about the Unionist parties failing to deliver on ‘The Vow’ — the importance of which he has blown out of all proportion, to suit his agenda — it is the Nationalists who are not to be trusted. They are not Scotland’s champions — they are champions of an independent Scotland, which is a different thing. They will not go to Westminster looking for the best deal for all Scots, but to feed the panting fervency of ‘the 45’ and to demean the Union. There is no evidence, nothing in their history, to suggest anything else.

So if you’re a Unionist, think carefully about what you do on May 7. Sturgeon is a more alluring individual than Salmond — who isn’t? — but that only makes her a greater threat. If you want to avoid the chaos that will come with putting those who would destroy the British state at its very heart, use your vote to stop it happening. If it looks like a separatist could win your seat, back the candidate with the best chance of beating them — you can find the information you need at http://www.scotlandinunion.co.uk . This is, to all intents and purposes, a second referendum — and it is the SNP that has made it so. Let’s not lose it.