WHAT a swell party that was. Judging by the pictures of Scotland manager Gordon Strachan and his wife Lesley celebrating their ruby wedding anniversary in Spain with Rod Stewart and other guests, a grand time was had by all out in La Manga. Forty years, eh? The honeymoon between Scottish voters and the Scottish Conservatives has barely reached the 40-day mark and already the union has run into trouble.

The cause, alas, is a third party. Cast your mind back to June 9, when the Scottish Conservatives were celebrating remarkable wins in the General Election. From just one MP to 13. Where’s your panda jokes noo? Their leader, Ruth Davidson, was held up as the very model of a modern Tory major-general. Everyone wanted their photo taken with her; no longer did she need to straddle buffalo or tank guns, the political planet that is Westminster was hers to bestride like a colossus. Fancy a peerage Ruth, cooed the commentators? Drop into Cabinet any time said her ministerial colleagues. The kettle, like the leadership election, is always on.

Not that those Scots voters who had placed their cross by the Conservatives were left feeling out in the cold. They could warm themselves with the notion, oft repeated by Ms Davidson, that they had stood up to the SNP and defeated all plans for a second independence referendum They had taken back control, but in a good, non-Boris and Farage way. Theresa May’s narrow victory looked like it could work in Scottish voters’ favour. It was the baker’s dozen of Scottish Tories, making up for losses in England, that had kept her in business. That had to count for something.

Alas, no. A better offer came along in the shape of the DUP and its 10 MPs. These Dr Nos will say yes to the Budget, the Queen’s Speech, Brexit legislation and boosts to security, and all for the bargain price of £1 billion. The extra cash will put spending per head in Northern Ireland at £14,263, compared to £13,054 in Scotland, £12,531 in Wales, and £11,297 in England.

It would perhaps be harsh to say that voting Tory in Scotland has turned out, like the vice-presidency of the US, to be not worth a bucket of warm spit, but it is definitely beginning to look like the equivalent of buying a “solid gold watch” from a seaside auctioneer. Stopped working five minutes after you bought it, mate? Turned your wrist as green as Patrick Harvie, love? We would care, but we’ve pocketed the money and scarpered to Westminster. Take it up with trading standards. I think you’ll find any bargain with a politician comes with a dirty great “caveat emptor” warning.

Ms Davidson has been taken for a mug by her UK party leader and no mistake. She and her band of MPs at Westminster can point out till they are as blue in the face as their now packed-away rosettes that there is nothing unfair in giving £1 billion extra to Northern Ireland and sending zip to Scotland or other parts of the UK. Governments have made special payments before, after all, including £500 million for Glasgow. Moreover, under the Barnett rules, it is only a change in spending in England that has to be mirrored in Scotland, Wales and Northern Ireland.

But Ms Davidson will need all the luck of the Irish in making those defences stand up in Scotland. Impressions are everything in politics and most people will regard Mrs May as a weak, misguided, selfish leader who has bought the support of the DUP so that she can stay in Downing Street. Everything else, including a souring of relations between the nations of the UK, the effect on power sharing in Northern Ireland, and the sheer jaw-clattering unfairness of such a deal, are of secondary consideration. What price principle? One billion will do nicely, thank you.

When it comes to defending Mrs May’s actions, Ms Davidson might like to start with the Scottish Secretary, David Mundell, who said last week that if money went to Northern Ireland then Barnett rules would ensure “the appropriate funding comes to Scotland”. Perhaps Mr Mundell did not get the memo from Downing Street. Did Ms Davidson? Was she consulted over the DUP funding deal? She was quick, after all, to seek assurances on June 9 that if there was a pact with the DUP it would not have a detrimental impact on gay rights outwith Northern Ireland.

Ms Davidson might like to believe that the dust will settle and the honeymoon between the Scottish Conservatives and voters can pick up again. She is kidding herself. The fun and games have only just begun. The deal is for two years, after which it will be “reviewed”. If Mrs May makes it that far without the DUP coming back for more, then a third runway at Heathrow open to pig traffic must surely be close behind. And every time the pact is mentioned, Scottish Tory voters will wonder anew if they bought a pig in a poke.

For further readings on the implications of the DUP deal one turns to those who are often the fiercest, most perceptive, critics of the Conservatives: no, not the SNP, Labour, the media, or any other relative amateur, but Conservatives themselves. As Michael Heseltine told the BBC’s World at One, the pact is not a long-term solution as by-election losses could erode the majority. Moreover, the former deputy PM added:“I don’t know how much actually has been promised under this deal, but whatever it is, what does Ruth Davidson say to the loyal Scots that she has persuaded to join the Tories?”

What does she say? Helpfully, there’s another Conservative lord to hand with his analysis. Chris Patten put the “hell of a mess” the country was in down to “the calamitous errors of two Conservative prime ministers in a row who thought they could manage the unmanageable English nationalist right wing of the Conservative Party”.

So there you have it, Ms Davidson. You can tell those “loyal Scots” mentioned by Lord Heseltine that your efforts to stymie Scottish nationalism have been grossly, perhaps fatally, undermined by your UK leader’s need to contain the English nationalist wing of your own party. What a swell time you will have doing that.