AWAY from the hurly burly of the latest Tory leadership crisis and Brexiter cabals, there was an interesting but overlooked development on the demise of devolution this week.

It didn’t have the drama of the SNP walk-out from Westminster seen before the summer, but it is likely to prove a more significant piece of obstructionism all the same.

Scottish ministers have told their UK counterparts that if they want Holyrood’s consent in passing any Brexit-related Bills they can get lost.

There is now an official policy of non-cooperation in place. You could call it legislative disobedience.

But this is not a fixed state of affairs. Holyrood could change its mind - for a price. But it is rather a steep price in London’s eyes.

The SNP wants a fundamental change to the British constitution.

The jemmy used in this plan is our old friend the Sewel Convention, which says Westminster will “not normally” stray into devolved issues without Holyrood’s consent.

Despite adorning the 2016 Scotland Act, this is not law, but an agreement between politicians. It cannot be enforced in court, and there is no sanction for breaching it.

The convention has only been departed from once since 1999, when Westminster passed the main Brexit law, the EU Withdrawal Bill, despite most MSPs objecting to it as a ‘power grab’ against devolution.

Westminster’s position was that Brexit is so exceptional, so “not normal”, that it had to be done.

The SNP cried foul and said the Sewel Convention was dead. But with only one example to cite, they didn’t sound very convincing. Now, however, more examples are coming into view, and the ground is shifting.

For instance, Edinburgh is being asked to consent to the UK Trade Bill. It’s not happening.

At a Holyrood committee on Tuesday, Michael Russell delivered his ‘non serviam’. The Withdrawal Bill had “in effect overturned 19 years of constitutional convention and practice,” said the constitutional relations secretary. “We are therefore seeking urgent discussions with the UK government on how to strengthen and protect the Sewel convention, and will be looking again at how we can embed the Scottish Parliament’s consent in law... In the meantime, we have made it clear that we will not introduce further legislative consent motions on Brexit bills.”

The next day he wrote to the cabinet officer minister David Lidington, telling him the Scottish Government “will not consider proposals” for common frameworks designed to keep the UK single market harmonised after Brexit where they touch devolved areas.

The “unprecedented and constitutionally improper” way the UK government ignored Holyrood’s view over the Withdrawal Bill meant “any decision on legislative consent by the Parliament is now effectively meaningless if it can be set aside by the UK Government.

“The confidence of the Scottish Parliament in the Sewel Convention, and thus its authority to determine the devolved law of Scotland, has understandably been undermined.”

Mr Russell then named his price for making nice again - upgrade the Sewel Convention into a hard and fast law. By happy coincidence, he was able to attached an annexe of proposed amendements to the 2016 Scotland Act to do just that.

The key would be to swap the ambiguity of “not normally” for a ban on Westminster passing Acts about devolved matters in Scotland “without the consent” of Holyrood.

That curb on Westminster power is anathema to the UK Government, and Mr Russell knows it. It would invert devolution by allowing a subordinate parliament to block the will of the sovereign one. That’s not a judgment on the right or wrong of the relationship between the two parliaments. It’s just a description of the situation as it stands. Mr Russell and his colleagues don’t like that situation and want to change it.

Now they’re stepping up their efforts. First you make an offer doomed to fail, then dig in, exploit the ticking Brexit clock, make an exasperated UK Government push ahead unwisely with laws affecting Scotland without Holyrood’s consent, rage against the injustice of it, cry ‘look the system is broken and we need a change’, and use it to sell independence. It’s not a bad plan.

But it’s not foolproof. Ministers are withholding consent without asking Holyrood. MSPs may not appreciate what is being done in their name.The more this looks like a Nationalist tactic, the less willing other parties will be to play along.

For instance, the SNP went straight to Defcon 1 this week when the Brexit-related UK Agriculture Bill was published. Rural economy secretary Fergus Ewing railed about it riding roughshod over devolution and vowed “not to bring forward legislative consent motions for primary Brexit legislation, such as this” until Sewel was fixed.

The next day, the Cabinet Office pointed out it wasn’t actually asking for legislative consent on the Bill, and issued a lengthy rebuttal to all Mr Ewing’s complaints. Awkward.

The accelerating Brexit process gives the SNP an extraordinary chance to leverage the legislative bottleneck at Holyrood to pursue its founding goal. To succeed, it can’t afford to be as ham-fisted and hungry-looking as it was this week.