Photo call or 'pivotal moment' - How significant is leaders summit?

John Swinney has claimed the UK is at a "pivotal moment of constitutional change" - ahead of a summit with the leaders of Plaid Cymru and Sinn Fein in Cardiff.

Swinney, Plaid's leader Rhun ap Iorwerth, and Sinn Féin's president Mary Lou McDonald and vice-president Michelle O'Neill, will discuss how the parties can work together on their shared interests.

The gathering marks the fact that, for the first time, the three devolved governments are represented by three nationalist first ministers (although O'Neill represents one half of Stormont's power-sharing administration).

All three seem keen to exploit this new political dynamic. But the question is, how exactly?

After all, when you dig beneath the surface of their shared goal of leaving the UK, the circumstances facing them look very different indeed.

Swinney is pushing for a second vote on independence following his win in May's Scottish elections.

In Wales, ap Iorwerth - who leads the Senedd's first pro-independence administration - is in less of a rush for a referendum, previously ruling one out for this parliament, and instead focusing on the devolution of more powers.

Speaking on Sunday with Laura Kuenssberg, ap Iorweth acknowledged the differing contexts.

"We have distinct nations with our own trajectories and our own priorities," he said.

"But there is also something that ties us together...that pursuit of a different attitude from UK government.

"It could be around the constitutional question, it could, more acutely, be around fair funding."

In Northern Ireland, there is already an agreed path to reunification as set out in the Good Friday Agreement.

That question of the devolved funding settlement is expected to be discussed, alongside other ways of working together in areas such as the economy and energy.

But perhaps the biggest opportunity for the nationalist leaders lies in the political rather than the practical.

Speaking ahead of the meeting, Swinney said: "For the first time ever, Scotland, Wales and Northern Ireland are led by pro-independence first ministers.

"There could be no clearer sign that the status quo is not sustainable – and that Westminster must prepare for a post-UK future."

O'Neill echoed those sentiments, describing it as a "clear indication that people see that Westminster is not working for the people of the North, Scotland or Wales".

A photo-op featuring three nationalist first ministers is certainly a very visible reminder of the recent shift in the UK's political landscape - "good optics" as they say.

And the prospect of the trio of parties signing up to a shared agreement - aimed at putting pressure on Downing Street to plan and prepare for referendums - is a media spectacle, even though it carries no real weight.

The timing of this summit is also a gift for the SNP.

At last week's PMQs, Andy Burnham seemed to accept a parallel with Northern Ireland that if, and when, support for constitutional change reaches a certain level, a referendum in Scotland could be considered.

Downing Street has since rowed back, but as one SNP figure puts it, "the cat is well and truly out the bag".

Swinney is now seeking an agreed mechanism for triggering a vote, and his aides believe events like this one in Cardiff can help build both support and momentum.

Meanwhile, President Trump's remarks in support of Irish reunification have ensured the UK's constitutional arrangements remain firmly near the top of the news agenda.

That could feel like a "momentum changer" to Irish nationalists, according to the BBC's Northern Ireland political editor Enda McClafferty.

Of course, today's meeting of minds is not without risks.

The SNP and Plaid Cymru have a history as allies, but Swinney has faced criticism over his approach to working with Sinn Féin.

The shadow Scottish secretary Harriet Cross has described it as "a performative photo call" and a "distraction", adding that Swinney "should be spending time focusing on what really matters to Scots".

And there are some in his own ranks who believe talk of a referendum should be put on the back burner in favour of driving up support for independence.

With polling indicating Scots are evenly split over the constitutional question, that remains Swinney's biggest challenge.