Analysis: Tony Blair let us into the secrets of the prime ministerial bedroom in his Edinburgh hotel on Thursday night. Flicking through the TV channels in search of football, he found himself aghast at an interview with Alex Salmond.

Analysis

TONY Blair let us into the secrets of the prime ministerial bedroom in his Edinburgh hotel on Thursday night. Flicking through the TV channels in search of football, he found himself aghast at an interview with Alex Salmond.

The SNP leader was claiming that Czechs and Slovaks had happily and easily parted, and that English and Scots could do likewise. "That is just so politically, economically, historically and culturally ignorant," fumed the Prime Minister. "I find it truly scary."

He was returning to his passionate belief that England and Scotland are better together. Conceding that Scotland could be independent and stressing the decision is for Scots, his message was being refined for a business breakfast audience to a simple question - why?' - and a challenge to voters - think carefully'.

The message was clouded by the intervention of Sir George Mathewson, the eminent Scottish businessman, backing the SNP and rubbishing the Labour proposition that Scotland's financial services would be left stranded behind the boundary fences of an independent Scotland.

Mr Blair's response was to get personal, attacking Mathewson as self-indulgent. That may not help next time Labour rolls out a celebrity or business backer.

This was a big day in the build-up to the May 3 ballot. Adding a visit to Aberdeen and an offshore oil rig, the Prime Minister was raising the stakes the day before the SNP conference opened in Glasgow, while the SNP played Mathewson as a surprise, high- scoring trump card.

The governing party stuck to its plans to publish a key part of the economic case it has been building to confront the Nationalists, reckoning that every Scottish home would face an annual bill of more than £5000 as the cost of independence. Labour has yet to make its own costings clear, but the SNP has counted £3bn in commitments so far.

So, how to explain yesterday's claim? Voters would be forgiven for having lost interest in recent months of statistical exchanges, but it is worth revisiting Labour's contention there is a "black hole" in Scotland's finances, meaning an £11.2bn gap between the money raised through tax in Scotland and the money spent by central government north of the border.

In response, the SNP argues Scotland is £3bn in surplus, based on a share of Gordon Brown's borrowing, or it sometimes says none of the figures are reliable. Even if the current position didn't look rosy, it continues, a business tax cut would give a dynamic, Irish-style boost to the economy and tax takings.

The black hole is the basis for former Enterprise Minister Wendy Alexander to step up her statistical warfare on Labour's behalf. She put the alleged deficit alongside the SNP's spending commitments, totted up costs against income, and found that Scotland's oil receipts would not dent the nation's deficit.

The calculations require some scepticism, as they would mean an independent SNP government introducing all its policies at once. It also puts contestable interpretations on what the SNP has and has not said.

Starting with the £11.2bn deficit claim, Labour adds policies the SNP would introduce if it got control of Westminster's reserved powers: pensions costing £1.2bn and a corporation tax cut of £800m.

Its devolved policies stack up, too. The SNP says it would pay off student debt at the same pace students would have done, but Labour insists that must happen in one go. Less contentious is an increase in pre-school education for £73m per year and free school meals for £40m.

The SNP has not explained how it would put a million wind turbines on Scotland's roofs but Labour takes the maximal approach and reckons on £1.5bn over four years. So, too, for schools, and with grants to help first-time buyers, at £272m over four years. The other big ticket item is the cost of replacing council tax with local income tax, while cutting business rates. The SNP has costed it at £1.3bn over four years. Labour assumes very differently and says £3bn.

The Labour reckoning takes an independent analysis of separate state institutions, ranging from an airforce to tax collection, totalling £1.5bn per year. And rather than setting this spend against the 90% of the UK's current oil revenue that Scotland could hope for, it follows SNP policy by siphoning half of that into a national trust fund.

That arrives at yet another "black hole" of £11.9bn, or £5242 for the average Scottish household. "We have not suggested that the full £5000 would be raised solely from further income tax hikes," Labour leader Jack McConnell said yesterday. "There are many ways the SNP could tax Scottish families. But there's no doubt that whatever taxes were used, the SNP's tax bill for your family would be £5000 more."

Actually, there is plenty of room for doubt. But in the battle between Labour and SNP, perception can be as important as fact, and the perception of a fat tax bill can do a lot of damage to the Nationalists.