The question on everyone’s mind at Westminster since the SNP dramatically walked out of the Commons chamber and declared ‘guerrilla war’ on parliamentary niceties has been: what’s next? And just how bloody will it get?

We got the answer yesterday. SNP MPs did what any well-drilled insurgents would have: they seized the airport.

By abstaining in yesterday’s vote on a third runway for Heathrow, the SNP demonstrated just how committed they are to their campaign of disruption. But, in taking no hostages, this strategy could backfire.

A composite image of planes taking off from Heathrow Airport (Picture: Getty)

This vote wasn’t like previous occasions when the SNP group have taken the UK Government’s agenda hostage, on Sunday trading or fox hunting in England. Nor can Heathrow easily be painted as an expensive, publicly funded infrastructure project that promises much and delivers little north of the border, like HS2.

Private sector investment in Heathrow is set to bring significant economic benefits to Scotland, including in the region of 16,000 additional jobs. The figures were accepted by the Scottish Government when it backed a third runway and signed a memorandum of understanding with the airport’s owners two years ago – in full knowledge of the environmental implications.

Scotland '˜would really benefit' from third Heathrow runway

As late as Sunday night, the Scottish Government was telling the BBC that it “supports expansion at Heathrow” and “looks forward to Scotland seeing the benefits outlined” in the memorandum. By Monday afternoon, a Scottish Government source was pointing out that it was also in favour of more direct international routes from Scottish airports, and talking about the “relative lack of detail on the supposed benefits”.

Economy Secretary Keith Brown hailed those “supposed benefits” in 2016 when he called on the UK Government to support Heathrow expansion, something his own party’s MPs have now failed to do. Scottish routes are already benefiting from a £10 per passenger discount on landing charges, and the Scottish Government secured a commitment to £200m worth of spending in Scotland during the construction phase, including the possibility of a ‘logistics hub’ at Prestwick Airport.

In addition, Dundee is already pencilled in as one of five new routes that will benefit from a £10m fund to improve regional connections – to say nothing of the thousands of supply chain jobs. All of these are subject to delivery – but it’s hard to argue that the deal isn’t a good one, mostly because the Scottish Government said so, two years ago.

In fact, it’s so good that Welsh political parties have used it as evidence to argue they are missing out because there are no new routes planned, despite Cardiff losing its only London connection last year, and no commitment to construction spending in Wales. As a result, Plaid Cymru voted against expansion, and even the Welsh Conservative leader Andrew RT Davies joined the attack on the Labour administration in Cardiff for getting “no strong and tangible commitments like those secured by counterparts in Scotland”. As of last night, Heathrow has less reason to honour its commitments.

There will have been a robust debate within the SNP group at Westminster, and you have to feel for the losing minority. What will Drew Hendry, the SNP MP for Inverness, now say to his constituents? He campaigned for reinstatement of flights to Heathrow and London for 19 years, and only this month, Hendry was hailing the “real increase in visitor numbers to the region” since their return in 2017, with “an international tourism boom like the Highlands has never before seen” to follow from Heathrow expansion.

It will be too easy for Unionists, who collectively outpolled the SNP in 2017, to argue that Hendry has given up representing his constituency to play political games at Westminster and agitate for another independence referendum.

SNP MPs clearly feel there is more value in appealing to the party’s core of the hyper-disillusioned with Westminster, and defending its left flank from the Greens and Labour. If they’re right, it would say a lot about where Scottish politics is and how much it has changed since 2016.

But it will also be easy for the SNP’s opponents to suggest that Nicola Sturgeon has lost control of her MPs at Westminster, and that their guerilla offensive against post-Brexit Westminster has become a scorched-earth campaign – with Scotland’s economy paying the price.

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