EurotaxGlass's, publishers of used car value guides, this week said there has been a surge in sales of new convertible cars in the UK during 2005, driven largely by growing demand for vehicles with a folding metal roof.

In 2001, new convertibles took 2.4% of the UK new car market, but by last month their share had climbed to 5.2%.

One bi-product of the rapid rise of cars with a folding hard-top - often referred to as coupe-cabriolets or CCs - is that convertible sales are slowly becoming less concentrated around the summer months.

"For conventional soft-tops, like the BMW 3 series convertible, Audi A4 cabriolet and Mazda MX-5, March remains the key selling month, with the last three months of the year the quietest," said EurotaxGlass's spokesman Adrian Rushmore.

"However, while March is still the high point in CC retail activity, these cars are widely perceived as more of an 'all-weather' choice, so registrations are more evenly spread throughout the year."

Despite the inevitable increase in availability of convertibles now entering the used car market, there is no evidence of any weakening in residual values relative to other sectors. The average volume-brand convertible retains 52% of its original list price after three years and 36,000 miles, compared to 59% for a premium brand model. This compares very favourably with the 45% retained value for the average used car across the market as a whole.

While all three figures are down compared to this point last year, the relative differences between the three categories are unchanged, demonstrating the continued strength of convertibles in the used market. It is, however, apparent that the residual values of CCs are starting to outperform those of conventional soft-tops.

"Over the next two years we expect to see values of three-year-old soft tops from the volume brands lose ground to the CCs," noted Rushmore.

"Volume-brand coupe-convertibles such as the Peugeot 307, Renault Mégane CC and Vauxhall Tigra are likely to appear in ever-increasing numbers, and should achieve higher residual values than soft-top alternatives. The convertible car market still has a lot of potential in terms of expanding new sales and there will remain a ready market for used examples."