Gordon Brown has finally joined the electioneering. The centrepiece of our manifesto," he declared last night, "will be the economic future of Britain".

It would be a funny election if it wasn't. But since Mr Brown and his Cabinet colleagues have yet to sign off the manifesto, there was nothing he could say about it. Instead he announced the British living standards have risen by "the order of 20 per cent" since Labour came to power eight years ago.

That sounds good. It is good. Strangely, for the last couple of years Mr Brown has been basing his Budgets on something better.

The British economy is generally supposed to grow at a "trend" rate of 2.5 per cent --which compounds out as 22 per cent over eight years. Recently Mr Brown has been going for 2.75, or up to 3.25 per cent. It has worked out, too.

The interesting bit is how much longer it can go on working out. So it is intriguing that our Chancellor started his campaign leaving no hostages to fortune. Why should he? Percentages don't swing elections.

Earlier yesterday Nationwide provided a reminder of how hard it is for a Chancellor to make voters happy without spending serious money.

The building society examined his move to raise the starting point for stamp duty on house purchase to £120,000 from £60,000. It looked clever, a saving of up to £1,200 for tens of thousands of individuals at a moment in life when most of us feel poor.

The catch is that the average price for a first-time home is already £125,498 - above the new threshold. So, because stamp duty applies to the whole price the moment you cross the trigger point, the average first-time buyer won't save a penny.

True, it works better in the West Midlands where the average first- time price squeaks into the tax-free bracket at £116,851.

Nationwide calculates that while 94 per cent of the region's home-buyers paid stamp duty last year 57 per cent will now - and only 34 per cent of first-timers - if the housing market replicates its 2004 performance.

Behind the percentages, Nationwide reckons that more than 8,000 people in Birmingham will benefit. They will be pleased. But the electoral impact of their pleasure depends on how many of them would vote Labour anyway.

Then you have to set them against a much larger number of home-buyers who miss out, many of them narrowly. They will be displeased.

The rub with stamp duty is not that the starting point is too low - it is, to restore the position when Mr Brown became Chancellor he would have had to raise the threshold to £185,000.

It is that it is graduated in steps which apply to the whole price, to, as with income tax, only to the amount above the step. It distorts the market.

You won't find anyone this agreeing to pay £120,001 for a house. That final £1 will cost the £1,200.

A remarkable number of homes change hands at just under £250,000, the point where the rate jumps to three per cent.

Chancellor Brown could have won more friends if he had done something about that.