AS Alex Salmond's ministerial car yesterday swept up the mile-long drive leading to the Northern Ireland Assembly buildings at Stormont, he brought with him tumultuous change.

This was statesman Salmond on his first official trip outside Scotland as First Minister, meeting his Unionist opposite number the Rev Dr Ian Paisley and his Sinn Fein deputy, Martin McGuinness.

Rather than take his case direct to London, or to reach out to small independent nations, he was there to build alliances within the UK and to show he is not alone in driving a new constitutional road which could yet prove bumpy.

People are still blinking in astonishment that Mr Paisley and Mr McGuinness are willing to share the same room, let alone a government and some public camaraderie. Alex Salmond only adds to the air of disbelief, providing significant symbolism that there is a new alliance finding common cause on the so-called Celtic Fringe.

There may have been a different symbolism for Ulster unionists, among whom the critics of Mr Paisley painted this as a Scottish-Irish nationalist trap.

But it is a remarkable change for Alex Salmond as well, who has previously been desperately keen to avoid any link between his Scottish brand of constitutional nationalism and that of Sinn Fein's.

Being seen anywhere near Martin McGuinness is yet another sign that the province's politics are becoming safe for outsiders, and that the SNP is relaxing into its new role.

The main common ground is over the Treasury in London. Northern Ireland's First Minister and his deputy found common ground in demanding a big Ulster payout as reward for their deal.

Plus they have been putting pressure on Gordon Brown, as Chancellor, to give the province a level playing field when trying to do business alongside the Irish Republic's corporation tax rate of 12%. With the UK rate at 28%, that means multi-nationals are avoiding Northern Ireland and crowding into the south.

The SNP has been making the same point, though it is less obvious that the Republic of Ireland has been costing Scotland jobs. Mr Salmond has positioned his party as pro-business on the back of the Irish example of driving growth through low taxes.

And Mr Brown knows that if he could get round European tax rules to make a concession for Northern Ireland, as a peace dividend and effectively conceding a measure of fiscal autonomy, he could be assured of a demand from his SNP adversary for Scotland to get similar treatment.

For Mr Salmond, Stormont talks showed this is not all about common interest. His new Ulster allies want transport improvements through Stranraer and demanded that their students should not be charged higher fees to attend Scottish universities than those from the Republic and the rest of Europe.

The cutting of tuition fees left an anomaly that students from England, Wales and Northern Ireland faced discriminatory fees to dissuade them from swamping lower-priced Scottish universities, while all other Europeans face the same fee levels as Scottish-based students.

The message to Westminster from yesterday's meeting should be clear, though it may seem confusing when viewed from the banks of the Thames, where power-sharing at Stormont is seen as one of Blair's greatest achievements and Alex Salmond's election as one of his worst failures. They will be even more confused if they look to Wales.

Rhodri Morgan, the man Blair tried to block from leading Labour in Cardiff, returned to office last month. Having experience of running a minority administration before the May 3 election, he remains keen to do a full coalition deal and secure a majority.

Talks with Liberal Democrats failed, but he is now exploring the possibility of forming a partnership with Plaid Cymru. The party may not be for full independence from the UK in the same way as the SNP, but such a grand alliance across the main constitutional divide in Welsh politics might get Scots thinking whether a similar move might work in Scotland.

This changing shape of devolution is among the range of constitutional challenges Tony Blair leaves his successor as he departs 10 Downing Street next week. Such reforms are among his notable achievements. But on almost every front, they remain half-finished.

Controversy has returned over the future of the European Union constitution, which the outgoing Premier leaves up in the air. House of Lords reform could sidetrack the new Prime Minister, Gordon Brown. The tensions between anti-terrorism measures and civil rights have yet to be resolved. The Supreme Court is still to be set up, and Mr Brown has already indicated he wants to give Parliament more powers, at least over declarations of war.

Under Blair, Westminster has been slow to adapt to the constitutional changes around it. It has watched with alarm as the different parts of the UK have embraced policy differences that cause domestic trouble in England, from free prescriptions in Wales to free personal care in Scotland, and the abolition of the Scottish graduate endowment sparking new irritation in England, with its top-up fees, that the Scots appear feather-bedded.

One significant sign that Westminster may be adapting to change came yesterday from David Miliband, the Environment and Rural Affairs Secretary, hosting a meeting with representatives of the devolved administrations.

His Downing Street colleagues may have been reluctant to acknowledge the election of SNP ministers, but he has to work with such political opponents on the European agricultural and fisheries council and on cross-border environmental policy, including the Marine Bill.

It has not been lost on the Labour minister who was closest to challenging Gordon Brown for the leadership that there is radical change afoot in governing Britain and he can position himself at its cutting edge.

Mr Salmond is pushing for the formal mechanism for sharing information and resolving disputes to be set up.

These Joint Ministerial Committees (JMCs) were part of the 1999 devolution settlement and started sharing experience of tackling drugs and poverty. But from 2002, any such links were carried out by officials or through Labour Party channels, and only the European JMC has met since.

Being mechanisms of the Labour government's own devising, it is hardly a radical plan for the SNP administration to ask for them to be reconvened. And as demonstrated by the row over Tony Blair's memorandum of understanding with the Libyan government over prisoner transfers, there will have to be some methods set up for avoiding clashes or resolving disputes.

While refusing to see that row as a mistake, at least in protocol, a Whitehall source told The Herald the JMCs will probably return, but the summer recess and Gordon Brown's transition to PM means it is unlikely before October.

From America's state governors and city mayors lobbying as one group in Washington to Germany's regional Lander having well-staffed "embassies" in Berlin to lobby and work together, this is nothing new for other countries where it is normal for power to be shared between layers of government and exercised by rival parties.

But this is new to Britain, and means Gordon Brown is going to have to join Ulster Unionists, Sinn Fein, Plaid Cymru and Scottish Nationalists in living a bit more dangerously.