THERE was something oddly inspiring about those archive shots of Labour’s Donald Dewar, the Scottish Liberal Democrat leader Jim Wallace and an impossibly young-looking Alex Salmond uniting on the steps of the old Assembly building on Calton Hill 20 years ago. Did they really set aside their differences in the campaign to restore Scotland’s Parliament after 300 years? They surely did; pity there isn’t much of the same spirit around today.

The response to Nicola Sturgeon’s call to arms in defence of Holyrood’s constitutional status has been decidedly lukewarm, if not curmudgeonly. Why doesn’t she give up her obsession with independence, critics say, if they want our support? She should get back to the day job and so on. I don’t recall Mr Dewar demanding that Mr Salmond abandon the SNP’s raison d’etre before he would accept SNP backing for the Scotland Bill.

The Tories, who are supposed to support devolution these days, accused the First Minister of stoking up grievance through nationalist trickery. Commentators say she is cynically exaggerating the threat to parliament posed by the Great Repeal Bill. If she’s exaggerating she’s in good company.

It was the truculent Nationalists on the House of Lords EU Committee who reported in July that the EU Withdrawal Bill (EUW) was technically illegal because, under the Scotland Act 1998, “powers notably over agriculture, fisheries and the environment should fall automatically to the devolved jurisdictions at the moment of Brexit”. The Law Society of Scotland said this week that, even in non-devolved areas, the bill could “remove the legislative competence of the Scottish Parliament”. And it was the Labour First Minister of Wales, Carwyn Jones, who first warned of “a Westminster power grab”.

If this isn’t a time to raise concerns about the future of Holyrood’s powers, I don’t know when is. Non-Nationalist supporters of home rule must surely see that the Great Repeal Bill is in danger of becoming the Repeal (Devolution) Bill. This is because, as those legal authorities have argued, Theresa May is reversing the relationship between Holyrood and Westminster, enshrined in the Scotland Act. Hitherto, only those powers reserved to Westminster in Schedule 5 are specified and Holyrood is assumed to have competence where it is silent; but not any more.

When those 11,000-odd pieces of Brussels legislation come back it is the Prime Minister who will hoard them and decide, under her ludicrous Henry VIII powers, which ones are permitted to find their way back north. That in itself is a “fundamental challenge” to our constitution, according to the Lords (none of whom are from the SNP). We’re not just talking agriculture subsidies; there is a whole range of measures involving the environment, food standards, justice and employment law.

Voters find constitutional issues abstract and frustrating; it was exactly the same in 1997. A lot of people didn’t really get devolution; all the stuff about bringing power closer to the people. What was the point of a “pretendy parliament” as Billy Connolly called it? But 20 years on, no one could seriously argue that the Scottish Parliament failed to deliver on Mr Dewar’s promise of “Scottish solutions to Scottish problems”.

The Scottish NHS has not been privatised, education remains defiantly comprehensive and free from selection and the right to buy has been abolished. Holyrood salvaged the universal principle in higher education. You need only observe the disastrous impact of tuition fees to see what might have been here; similarly with personal care, the smoking ban, free prescriptions, land reform, climate change targets and electoral and prison reform.

Attitudes to gender equality and sexual minorities have been transformed since the days of Keep the Clause. Could anyone imagine Ruth Davidson becoming Scottish Tory leader had it not been for devolution?

Few of these achievements were down to any one party. The Liberal Democrats first sought to abolish upfront fees and replace them with a graduate endowment, which the SNP scrapped. It was an SNP member’s bill that began the debate about the smoking ban, implemented by Labour in 2005.

Free personal care was introduced by Labour but it was the SNP that defended universal benefits against subsequent attacks on “the something-for-nothing society”. I’m not saying Ms Sturgeon, Alex Rowley and Willie Rennie should have a group hug and sing Kumbaya. But they need to recognise their collective responsibility at a turning point in the history of home rule. Instead of bickering, our political leaders need to turn the Brexit bill into an opportunity to defend the powers of the Scottish Parliament and extend them; to deliver more Scottish solutions to Scottish problems.

This is not a Nationalist argument or even necessarily an anti-Brexit one. The parties should see the Bill as a chance to entrench the powers of the Scottish Parliament. The Sewel convention, which prevents Westminster legislating on Scotland’s behalf without consent, should be placed on a statutory footing, without the weasel words in the 2016 Act that allow it to be set aside.

Amendments already tabled to the EUW Bill in the Commons and the Lords should ensure at the very least that there is co-determination, to use a Brussels term, between the Scottish Parliament and Westminster in deciding the destination of repatriated laws. Promises made before Brexit about funding and powers over matters like agriculture and fisheries should become Holyrood responsibilities, not on loan from Number 10, but there as of right.

This is not grievance-mongering or alarmism but reasonable demands which all of Scotland’s parties should endorse. If the UK Government rejects them, Holyrood and the Welsh Parliament must withhold consent to the Bill until Mrs May sees sense. It would be a tragedy if the Scottish parties allow tribalism to frustrate a united front on home rule. The chance won’t come again.