IT was a handshake of equals.

When a tanned David Cameron stepped lightly up the steps of St Andrews House in Edinburgh to be greeted by Alex Salmond, emerging on to the front portico like a powerful laird graciously welcoming a guest, they both affected an air of conviviality and good cheer that was almost convincing.

Mr Cameron's smile outside the Scottish Government's headquarters was perhaps just a little too forced and the First Minister's blokey pat on the arm to the Prime Minister just a little too unlikely, but they gamely kept up the facade.

The aim for both was to present an image of statesmanlike gravitas, on the latest "historic day" in this constitutional marathon. It might have worked, too, if the brief handshake hadn't been lent an air of farce at the last moment by a member of the public loudly booing from behind the security cordon. Cue a swift end to the photocall.

Not to worry, though, there were more pictures to be done of the document signing and then a press conference to be held by the First Minister and his deputy, Nicola Sturgeon.

There, Mr Salmond maintained the statesmanlike demeanour. There was less of the easy grin that he wore in January during his Government's referendum consultation launch at Edinburgh Castle, and fewer jibes at his opponents. The independence cause is faring less well in the polls now than it did then.

The process agreed, this was the start of a bid to win the arguments in favour of independence. "If you win arguments, you win referendums," he stated, repeatedly.

It was also an occasion to stress his personal conviction that the referendum could be won. At points, he appeared to remember himself, standing up straight and looking his questioners straight in the eye, without even the merest shadow of a smile. It was almost as if he had been urged to avoid any air of smugness – which, it turned out, he had. "Don't look triumphalist, it says here," he quipped, referring to his press team's briefing, adding that of course he always listens to his advisers. "You've failed," someone hollered.

Behind the quips, however, it was a day of dramatic significance. Looking around the room, a who's who of political correspondents from north and south of the Border, the importance of the occasion was clear, as was its symbolic significance outwith the UK.

Shoppers on Princes Street were apt to be accosted by a young man with a thick Catalan accent asking them for their views on independence. Later, standing in the cold waiting for the Prime Minister to arrive, Marc Vidal of the Catalan paper Ara, one of several foreign journalists in attendance, predicted that the image of Cameron and Salmond would grace the front cover of every Catalan paper this morning, a sign of how what Mr Salmond calls "Scotland's Home Rule journey" remains of interest internationally.

With two years of that road still to be travelled, however, it remains to be seen whether the Scottish public's enthusiasm for the debate will last the distance.