THE Nigel Farage circus rolled back into Edinburgh yesterday to be greeted by vocal protests on one side and crazy promises of armoured cars to protect him on the other.

The last time he visited, a shambolic bar room press conference descended into farce - and nearly worse - when demonstrators hounded him along the Royal Mile and back into the pub, where he was locked in for his own safety. It was hardly any wonder this European election campaign launch - the reason the Ukip leader was in town, in case you hadn't noticed - attracted more press attention than the SNP, Labour and Conservative efforts combined.

The truth is, however, that Mr Farage has been stirring up Scottish politics for quite a while without having to open his famously large mouth north of the Border. His party, which is chaotically disorganised in Scotland, is not active in the independence debate but, like a good old-fashioned political football, it is being kicked about enthusiastically by the Yes and No camps.

The SNP and the wider Yes side see the rise of Ukip down south as evidence that Scotland and England now have radically different, alien even, political cultures.

Scotland is a land of liberal values and left-of-centre politics, the narrative goes, while England is in thrall to the anti-EU, anti-immigration views of Mr Farage. Even if he doesn't actually have a seat at Westminster, it's argued, the pressure he is putting on the other parties is dragging them all to the right. David Cameron's promise of an in-out referendum on Britain's EU membership was a response to the Ukip threat and could result in Scotland being removed from the bloc against the wishes of the majority. With all that hanging in the air, it's time to leave the UK, the Yes argument goes.

The No side has a very different explanation for the rise of Ukip. Labour, the Conservatives and the LibDems believe the party is England's SNP. Ukip performs poorly in Scotland, they believe, because it has struggled to break into a crowded four-party political landscape where its natural niche is already occupied by the SNP. Ruth Davidson made the point this week when she launched the Scottish Conservatives' Euro election campaign. Voters looking for a "bellicose, nationalist, anti-Westminster" voice were already well catered for, she said. She didn't need to mention the populist agenda and the high-profile leader; it was clear what she meant.

Neither argument is entirely convincing. The idea England is so right-wing it is politically unrecognisable when viewed from the top of Hadrian's Wall was undermined on Thursday by the Core Cities Group, representing England's eight most economically important centres outside London. Their visit to Glasgow warning that "greater separation" from Scotland would damage efforts to co-operate on economic development also served as a reminder that seven of the eight cities are Labour-run. In some cases, Labour-dominated, without a single Conservative councillor. All were in the North or Midlands, with Bristol, under no overall control, the exception geographically and politically.

The reminder chimes with a report last November by Stephen Low, of Unison, which found public opinion north and south of the Border has been strikingly closely aligned for years on a range of social and economic issues. Across the UK, Labour remain favourites to win the next General Election, though a poll last weekend suggested Ed Miliband's party was trailing the Tories in the south while consolidating its lead in the north. The evidence seems to suggest that if there is an ideological dividing line across Britain, it should be drawn from the Severn to the Wash rather than along the Scotland-England border.

But the portrayal of Ukip as England's SNP is also flawed. The SNP is fervently pro-EU and proposals to encourage immigration are at the heart of the economic argument set out in its White Paper on independence. Also unlike Ukip, it is highly professional and can no longer be seen as a party of protest, having stood at the last Holyrood election on its record in government. The differences couldn't be starker.

What's really propelling Ukip is altogether harder to fathom than either side in the referendum fight would have you believe.