One of the most valuable skills is the ability to value a car realistically based on its condition.

That means having a clear understanding of desirability and to what extent other factors impact on value. CAP now includes in its research price-influencing factors like colour, trim, optional equipment and history.

This research reveals the variation in impact of various factors, depending on model. For example, a missing service history will reduce the value of a Land Rover Discovery 2.5 Td5 XS Auto, registered in May 2003, with 32,000 miles by an average £940, while an incomplete franchised service history reduces it by £395.

Interestingly the latter is still more desirable to dealers than a full service history carried out by non-franchised outlets, which drops the price by £440. Ownership history is also important, with three previous owners reducing the Discovery’s value by £325 and one-time use on a rental fleet by £120.

However, three or more previous owners for an Audi A6 1.9 TDi SE approaching six years old, with 158,000+ on the clock, only reduces the car’s value by £120; previous use as a rental car cuts the price by more than the Discovery, at £135. The largest negative impact comes from use as a taxi, reducing value by £260.

A four-year-old Citroen Xsara Picasso 1.8i 16v SX, with 44,000 miles, is reduced in value by only £140 with no service history. Former use as a taxi knocks £370 off but ex-rental examples are only penalised by £95.