Ewan Crawford: Will Boris win a Yes vote for the SNP?

Before the phone hacking scandal took off, the coming months had seemed certain to bring about a fresh round of anonymous briefings, stories about leadership plots and a general airing of Labour dissatisfaction.

Having worked for a political leader who was under pressure, I am all too aware the politically lazy summer can be full of danger because of the lack of alternative stories to compete with a juicy period of speculation over a supposed leadership crisis.

But Mr Miliband's aggressive handling of the hacking issue has won him some time. Whether he can use this reprieve to convince voters he is a credible Prime Minister in waiting is doubtful but he has, at the very least, given himself that opportunity.

For political parties it is important to recognize that this scandal has not primarily been about press ethics, media ownership or illegal behaviour. Instead it has been about party advantage and how to secure it.

If anyone is any doubt about that they only need to look at Scottish Labour's rather desperate attempt to drag the SNP into the story based, apparently, on lingering pique at The Sun's support for Alex Salmond in the Scottish Parliament election.

The lameness of that manoeuvre doesn't, however, mean that the fall-out from phone-hacking will not have an impact on Scotland. In fact how this story develops, and crucially the consequences for the Westminster parties, could have a profound influence here.

Since the SNP victory in May various arms of the Westminster government have demonstrated real interest in the coming independence referendum. The Scotland Office, as you might expect, has engaged, but the recent announcement from the Ministry of Defence on the future of the armed forces was also made with a clear eye on the campaign.

Paradoxically, however, it is the decisions that will be taken in London without overt reference to Scotland that are likely to have the biggest impact, and among the most important is the question of the leadership of the two main parties.

If Ed Miliband has had a good crisis then clearly David Cameron has not.For the first time since he took up residence in 10 Downing Street, serious questions have been asked about his future.

In the short-term, the government has tried to ease political pressure by establishing a plethora of inquiries. While a useful tactic, the difficulty will arise when these inquiries start to take evidence, when leaks are inevitably reported and when findings are published.

Then there is the question of whether the Prime Minister's former press secretary, Andy Coulson, will be charged and whether he will face a criminal trial - an event that will make the media circus surrounding the Tommy Sheridan trial appear low-key.

Although clearly not under any immediate threat, and although none of the separate aspects of the story pose a real danger in themselves, it is the drip-drip effect that could be most damaging. Crucially, the Prime Minister might struggle to get across any alternative message if all he is asked about is phone hacking.

It is in this context that Boris Johnson has apparently launched his bid to become leader of the Conservative Party alongside his campaign to be re-elected London Mayor.

Mr Johnson has been an active commentator and player, not only in the phone-hacking scandal because of the role of the Metropolitan Police, and but also on the economy where he has been advocating tax cuts and an end to the 50p top rate.

At the moment this can reasonably be seen as part of securing high-profile national coverage in the run-up to the mayoral election next May. But it seems nave to assume Boris has not given a thought to any further elevation.

Indeed, far from being a ludicrous proposition, despite not being an MP he is already favourite to succeed Mr Cameron as Tory leader. The fact that he would have to find a way back into the Commons is clearly a complication but should his country come calling then presumably he may find it possible, like American senators who deny any interest in standing for president before announcing their candidature, to serve.

This is all of crucial importance to Scotland and to any independence referendum because it could help to focus the choice. The experience of Quebec during the independence referendum in 1995 demonstrates the importance of leadership.

In that campaign, the decision by the sovereigntists to give the charismatic Lucien Bouchard a key leadership role proved vital in whittling away a big lead for the No camp so that by the time the polls had closed Quebec remained inside Canada thanks only to a few thousand votes.

By contrast the Canadian prime minister, Jean Chretien, a Quebecer himself, failed to garner the same level of enthusiasm in making the case for the federalists.If, through events, Boris Johnson became Prime Minister of Great Britain, it seems fair to say he might struggle to be welcomed whole-heartedly by many people in Scotland, not just because of the curious public image he has created for himself but because of his right-wing political programme.

An issue to be considered by anyone voting in a referendum would then become - who would you prefer as your national leader: Alex Salmond or Boris Johnson?

Whether or not this scenario comes to pass it highlights a crucial area of weakness for any No campaign - who will lead it?

You might expect the prime minister of the day to head up any campaign on the constitutional future of the state. But whether that is a phone-hacking damaged David Cameron, Boris Johnson, George Osborne or Theresa May, the sheer unpopularity of the Tories in Scotland means any Conservative prime minister is unlikely to be the fore.

For Labour, despite the respite he has won for himself because of his performance in the last few weeks, unless Ed Miliband can find from somewhere the ability to convince people that he is a genuine national leader, his leadership has all the hallmarks of being short-lived and difficult.

When referendum time comes it is hard to make the case that the SNP will be facing a strong, vibrant UK Labour Party, focussed on the challenge of saving the Union. The party's underlying problems have been revealed in UK-wide polling this week, which shows more people still blame Labour for public spending cuts than the Coalition. The path back to power seems a long way off.

As far as the Scottish party goes, who knows who will be leader in two or three years time?

None of this should obscure the very real challenges facing the SNP as it gears up to fight the independence campaign. But it certainly helps when there is no sign, and at present, no real prospect, of a credible leader of the No campaign.

• Ewan Crawford is a lecturer in broadcast journalism and was private secretary to John Swinney when he was SNP leader