With the Greek crisis building and the aftermath of the attacks Tunisia in dominating the agenda in Britain, the latest PMQs in the House of Commons w‎as pretty small beer. There was a “can we all go on holiday yet?” feel about the occasion.

But there was one notable area in which Cameron made an impact, and that was in exposing the SNP to scrutiny as it demands ever more powers for the Scottish Parliament.

The SNP group of 56 has been pretty impressive since it arrived in Westminster, but eventually, once the excitement and the novelty wore off, the party was always going to face challenges.

Cameron got to the heart of it yesterday. When the SNP’s leader ‎at Westminster Angus Robertson rose to ask his questions of the Prime Minister he trotted through the SNP’s latest routine, complaining of betryal because even though the extensive extra powers on taxation and more are being delivered, the SNP needs to be able to say they are not being delivered in order to fuel a sense of grievance on which the SNP thrives. The party is also trying to pretend it is furious about English votes for English laws, which is a perfectly understandable measure in the light of decades of devolution to Scotland, Wales and Northern Ireland.

Incidentally, Robertson has until now done a great job of leading the Nats in London. He has done it far more effectively than Alex Salmond would have done, who now appears to be on an eccentric ego trip that has run out of control. The fashion in the upper reaches of the SNP, I’m reliably informed, is to diss Salmond as “very last year” and not at all in tune with the positive, engaging schtick that Nicola Sturgeon is using in a bid to move on from the divisive Salmond years that left Scotland a deeply divided country split right down the middle.

Anyway, Robertson got slapped down by Cameron pretty effectively. Cameron observed that the SNP’s weakness is that it is obsessed with asking for more powers but cannot say what it will do with them.

Several other SNP members tried similar questions to those posed by Robertson, and got nowhere too.

Cameron responded to all this with a better question. Which taxes and which welfare benefits does the SNP want to increase when Edinburgh gets the numerous new powers? ‎The SNP can never answer, because if Scottish middle-ground voters ever wake up to reality and realise that higher taxes are going to be involved, some of them may look for alternatives.

‎None of this will do serious immediate damage, but it is the line that the SNP’s opponents will push increasingly. And it cuts to the heart of the problem that has dogged Scotland for the last thirty years or so. For the Scottish Left and Nationalists it is always easier to concentrate on demanding new powers, to march and moan, than it is to get on with fixing the economy (facing market realities) and making the hard choices involved in reforming education and health ‎to improve life chances. That stuff will always be secondary to most Nats who are fixated on the constitution.

Iain Martin is Editor of CapX.