I rather enjoyed Peter Mandelson’s description of George Osborne as a schoolboy charging round the sweetshop taking sweets from every jar without a clue as to how he is going to pay for them when he gets to the counter.

He intends to reduce the deficit dramatically, cut taxes and protect frontline services. Mmmm. Sounds fine. And the tax cut element is all going to be paid for by ‘efficiency savings’ which David Cameron once described as a ‘trick’ used by politicians making promises they didn’t know how to fund.

Cameron, sounding more commentator than leader this morning, describes the intervention of a group of senior businessmen protesting at the planned National Insurance rise as a ‘significant moment in the campaign.’ I don’t blame him for that. There are some big business names on the letter to the Daily Telegraph, and he knows they probably carry more economic credibility than he or George Osborne do. But is it that significant? I would be very surprised to find any businessman in the land who would say ‘yes please’ to a proposed rise in National Insurance Contributions. Likewise, the vast bulk of them, presented with the choice between a tax cut or wasteful public spending will go for the tax cut every time.

So what the Tories have persuaded them to sign up to is a letter which says they would prefer cuts in wasteful spending to a rise in NICs. So would I. The question then is whether Cameron and Osborne really can make the ‘trick’ savings they promise, on the timescale set out, whether they can reduce the deficit to the extent they say it needs to be reduced to prevent Britain becoming Greece, (the gap between problem as analysed and deficit reduction as announced is enormous), whether they can fund all the spending commitments they and their colleagues have made, (gap between promises made and cash allocated is similarly gigantic) and whether they can do so whilst still implementing their Number One tax priority – a cut in inheritance tax for the richest families in the country.

So would people vote for a NICs cut? Yes. Would they vote for the eradication of waste in the public sector? Yes. But the Tories have not convincingly set out how any of the above would happen.

And I suspect some of the names on the list would also agree with the proposition that the Brown-Darling approach to the international crisis was the right one, and to a great extent prevented a crisis from becoming a calamity.

Their letter starts at the here and now. Fair enough. But we would not be here now if Cameron and Osborne had been making the calls back then. GB and AD have shown steady economic management in good times and bad. There is a still a credibility gap the Tories have not filled.

* Buy The Blair Years online and raise cash for Labour http://www.alastaircampbell.org/bookshop.php.