When you’ve been had by a really good con trick, it may take you years to realise you’ve been had. When a real master of the craft is involved, you may never realise.

Take, for instance, the Jam Auction. It’s a common one, this. You’ll find more than one in progress on the promenade in every cheap seaside resort. Maybe you’ve seen one. I have.

A crowd gathers as a slick salesman starts selling watches or electronic goods. He offers the first ten people with their hand up a watch for £3. He takes your £3, puts the watch on top of it and then says that he’s got an even better deal. If you give him a fiver, he’ll give you the “£50 watch” plus the £3. You will be taking home a watch worth a fortune, for just £2. In the end you buy a watch worth £1 for £5. A skilled artist keeps adding goods and money until you are out a fortune.

The Jam Auction is one of the tricks listed by the conman Simon Lovell in his book How to Cheat at Everything (“jam” is slang for a small-time con). It’s one of a number where victims are persuaded to buy back their own money at a loss. It all happens too quickly for you to work out what’s going on.

Lovell describes some colourfully named variants on the Jam Auction — the Matchbox Teaser, Buying a Brick, and so on — but being an American he is unaware of that peculiarly British scam — the Election Poster Trick.

This ruse is worked much the same way as the Jam Auction. It involves very fast patter, large amounts of your own money and goods worth less than they cost. It ends with you buying back your own money at a fat profit to the salesman.

Let me give you an example from the last general election. You’ll have to follow closely, or you’ll miss your money disappearing. In fact, this scam depends for its success on your attention wandering about half way through.

During the last campaign, the Labour Party put up a poster. “WARNING” it said, in large black letters against a yellow background. “The Tories will cut £35 billion from public services.” The Prime Minister and the Chancellor both stood in front of the poster to give the message an extra push. And despite getting into a well-publicised row about their figures with the very sharp BBC man Nick Robinson (at the time working for ITN), they both went on making the claim right through the election campaign. In vain did Mr Robinson point out that what the Tories were planning wasn’t a cut at all — merely increasing spending more slowly.

Here’s how the £35 billion figure on the poster was reached. The Tories had announced their spending plans all the way up to April 2012. Labour was a little more sketchy about what it intended to do so far in the future. Up until April 2008 they had firm spending figures; after that they had a couple of years of figures pencilled in, and for the final two years they hadn’t, quite sensibly, made any decision yet.

But they weren’t going to let such haziness get in the way of the Election Poster Trick. So they simply assumed that in all the years for which they hadn’t firm figures they would keep spending growth in line with the rate of growth of national income. Bingo! The difference between the Tory plans and this mixture of estimated and invented figures, was £35 billion (actually £34.5 billion, but what’s a half a billion pounds between friends, eh?).

Next week is the Budget. The Chancellor will announce figures all the way to 2011 and will pencil in numbers for the year after that. And what will we discover? That he is increasing public spending by much less than the estimate he used in the last election, and that using exactly the same bogus maths that he applied to the Tory figure, his budget plans could be described as a massive public spending cut. (The size? About £13 billion.)

The Election Poster Trick works by making everyone so confused about the figures that they can no longer distinguish between a cut and an increase in spending. Opposition plans to control spending are made to sound mad and threatening, Government plans to do the same as reassuring and inevitable. In the end you give £1,000 in stamp duty, get £50 back in pension credit or whatever and are told to be grateful for the Chancellor’s generosity.

Gordon Brown didn’t invent this trick, incidentally. He pinched it from the Tories. The £35 billion poster is just the 1992 Tory Tax Bombshell poster worked the other way round. The underlying trick is precisely, but precisely, the same.

And I think we’ve had enough of it, don’t you? We’ve now had at least four elections fought around ridiculous, synthetic tax and spending figures that illuminate nothing, and it’s time to call a halt. Of course elections should involve debate about the direction of tax and spending. But we don’t get that. Instead we have silly arguments about made-up figures. Next time we should resolve to do things differently.

But will we? Not if the Chancellor has anything to do with it. You can’t blame him. The great Election Poster Trick has been a big winner for him so far. Last weekend Mr Brown’s advisers were telling journalists that they hoped to use his new lower spending projections to “wrongfoot” the Tories, forcing them to look like deep cutters. In other words, in the same speech that reveals that one of his central election claims was utterly bogus, he hopes to set himself up to repeat it. I wish I could confidently tell you that this time no one will fall for it, but sadly I can’t.

So it is up to the Tories to avoid the trap. And their air transport package suggests they going to give it a try. Most of the coverage discussed the environmental consequences of the tax measures. But at least as important to the Tory leadership, I am sure, is the revenue they will bring in. The money will enable the Tories to announce cuts in some taxes without having to draw up one of those specious plans for tax and spending that have been centre-stage ever since John Smith’s Shadow Budget in 1992.

No more Election Poster Trick? That’s one way to improve the environment.

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