Open season for new-car hunters started on 1 August, with the now traditional new-number- plate rush. This frenzy of car buying is motivated mainly by snobbery; but, oddly, it benefits those too sensible or too poor to join in.

If you are in the market for a used car, now is the time to buy. Thanks to the law of supply and demand, more used cars mean lower prices. And behind almost every new-car transaction is a used car, a part-exchange that helps to fund the deal. All those M-registered new cars lead to a deluge of used ones from July to September, and a lowering of second-hand prices from which only a few upmarket models (mainly Mercedes, BMWs or high-quality Japanese- built cars) are exempt.

One of several fates awaits the trade-in. Good clean saleable cars with full service histories are usually retained by the dealer for resale, enabling him to profit on the sale of both the new and used car. Vehicles in a similar condition but not right for the dealer's forecourt will be traded on and resold at a more appropriate dealership.

The real dross, MOT borderlines often traded in for pounds 1,000 minimum part-exchange deals and low-rate finance, will be sold on or auctioned and then turn up in the 'bargain-banger' classified ads.

How can you take advantage of this used-car-buying climate? The simple answer is to shop around, be pushy and watch the prices tumble. I set off on a forecourt foray to find out what was on offer. To give the dealers credit, most could not have been more open about their offers. But I would have hoped for clearer information from one.

Top of the trade-in league are the Ford dealers, masters of the mass- market strategy. They have whole fleets being swapped for M-plates and are almost overrun with part- exchanges. The huge Dagenham Motors group, just across from the Ford works, promised in its ads an 'August part-exchange sale' that would be our opportunity to 'buy at trade prices'. The choice was suitably huge and the savings at first seemed impressive: for example, a second-hand L-plated turbodiesel Mondeo was marked down from pounds 12,995 to pounds 10,995.

That looked good on paper. However, the retail price of the same car, new and M-plated, is only pounds 12,725. And in the trade price guides, the suggested retail figure for a second-hand Mondeo, such as the one on offer, is pounds 10,700.

Derek, the salesman, confirmed this baffling state of affairs: ' pounds 12,995 is what we would have sold it for,' he said. Really? When I popped down the road and spoke to the firm's new-car salesman, he quoted the pounds 12,725 retail price (plus on-the-road costs) for the new car. I quoted that back at Derek, and he started back-pedalling, suggesting that it must have been a misprint, and that the first price for the second-hand car should have been pounds 11,995.

Over at Horncastle Ford, near Reading, better deals were to be had. They had a clutch of 1994 1.4 LX Escorts all pitched at pounds 8,695, about right in used retail price terms. After a chat with salesman Nick, however, I discovered that the price was going down to pounds 8,295 in a weekend promotion. I showed some interest in the nicest model of the trio, a 10,000-mile example finished in red, and Nick tempted me further with a price of pounds 8,100.

Of course, Ford dealers do not just have part-exchange Fords on offer, just as Vauxhall dealers do not just have Astras and Novas. I was attracted to a Vauxhall main agent, Jessups in Ilford, by an ad that said it had a collection of 'Other Makes at Clear-out Prices'.

What caught my eye in the list was a Toyota Corolla, a 1990 1.3GL model with a nice low 27,000 mileage, and an equally attractive pounds 4,995 price.

Trade value guides indicated that such a car would normally be priced at pounds 500 to pounds 600 more. So it was no surprise that there was a 'Sold' sticker on its windscreen. You need to be quick when there are genuine bargains around.

Most of the summer deals are on bread-and-butter models such as the Ford Escort, but occasionally there is something more interesting. Millennium, a Rover dealer in Finchley, had a Lotus Excel. According to the sales staff, the previous owner had part-exchanged it for a new Rover Cabriolet because his son could not get insurance for the Lotus. An Excel is not the most dynamic or desirable car in the Lotus line-up, but this was a one-owner, low-mileage (21,000), immaculate example, priced at a pounds 5,995. This looked a good deal, and not just to me: a gentleman in New Zealand had faxed an offer for it, and another part-exchange had found a new home.

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