The average prices of used diesel cars sold through the Autorola online portal rose 6.1% in Q2 as volume fell.
Their value increased by £681 to £11,839, helped by a fall in average age from 34 to 33 months and a decline in average mileage from 30,323 in Q1 to 26,112 miles. Used volumes fell 9.8%.
Average petrol car prices rose for the second successive quarter and were were £9,204, an increase of £226 (2.5%) despite a rise in average age from 31 to 32 months.
Petrol volumes grew (12.8%) for the third successive quarter to a three-year high.
The supply of hybrid and electric cars was minimal in Q2 with only a handful of ex-demonstrators hitting the Autorola portal at an average age of seven months and 3,000 miles. The average price of cars sold was £20,133.
Jon Mitchell, group sales director of Autorola UK, said: “The used market is still in rude health across all sectors. Diesel prices keep defying the market and continue to rise, while the supply and price of used petrols has never been higher.
"August may cool the market down after this long hot spell, but it looks unlikely prices will change dramatically over the coming months."