Customers looking to buy a one-year-old prestige-badge convertible or roadster will typically pay over £1,000 more in July and August than in December or January, says Glass’s.

Rates of depreciation for this type of car slow from February through to the peak spring and summer selling period. Then they lose value at a much faster rate from August through to the end of the year, says Richard Crosthwaite, prestige car editor at Glass's.

"For example, an '03-plated Mercedes-Benz SLK320 automatic will depreciate by £3,825 during 2004, but a significant 65 per cent of this depreciation occurs from August onwards," he says.

Crosthwaite says that owners interested in selling a prestige convertible will maximise their return if they seek a private or trade buyer in the months of April through to July.