Glass’s reports that many dealers have already begun to buy in prestige-brand cabriolets, in readiness to satisfy retail demand once warmer weather arrives in spring.

In previous years such buying has typically started in late February, but cabriolets were being snapped up as early as mid-January this year when the country was gripped by heavy snowfall.

Richard Crosthwaite, prestige car editor at Glass’s, said: “One reason for the sudden impetus is that stock levels are lower now than in prior years, following a decline in the volumes of dealer demonstrators and manufacturer management cars available to retailers.

“These two sources of nearly-new cabriolets have dwindled recently, as falling residual values led to curbs in registration activity.”

In fact, values of prestige cabriolets are at their lowest point in a decade, after several years of over-supply.

For instance, the value of the Audi A4 Cabriolet 1.8T S-Line CVT has fallen from 73% for a one-year-old example in February 2007 to just 56% this month for the same-age equivalent car. It’s a similar story for a one-year-old Porsche Boxster 2.7 manual, which has dropped from 78% to 64% over the period.

“Falls of this magnitude are unusual, and exceed the decline in values which occurs naturally during a product’s lifecycle,” said Crosthwaite.

Some prestige-badge models are now worth less in convertible guise than their coupe siblings, despite having a higher list price. The Audi TT Roadster costs almost £2,000 more than the fixed-roof Coupe when new, but a one-year-old Roadster is currently worth £250 less than the Coupe.

Looking ahead, Crosthwaite concludes: “It’s a little early to gauge how the cabriolet market will fare in 2010 but, with prices and weather conditions set to improve, the segment has got off to a very buoyant start.”