Independent used car dealers who want to help cash-strapped Londoners who'll be impacted by the expansion of the ULEZ area could do well by stocking selected cheap diesel cars with Euro6 standards which are ULEZ compliant, even though they were on sale before 2015.

While promoting car choice by age, as Euro 6 for diesel cars became mandatory in 2015 and Euro 4 for petrol cars was mandatory from 2006, makes it simpler for car buyers to understand, the benchmark for ULEZ compliance is the emissions standard, not the age.

Some car manufacturers were selling a few Euro 6 compliant diesel cars as early as 2012.

What this means is that if the car meets Euro 6 for diesel; mandatory from 2015, but actually implemented as early as 2012 – then it won’t be charged a ULEZ fee.

Similarly, under ULEZ rules petrol cars must meet Euro 4 (often styled as Euro IV), which became mandatory in 2006, but earlier models may be compliant too.

Colleagues at Parkers reveal that the best ULEZ compliant affordable second-hand diesel cars are:

• Audi A3 Cabriolet 2.0 TDI 150 from April 2014
• Citroen C4 Picasso BlueHDi 150 automatic, from November 2014
• Mercedes-Benz A 220 CDI BlueEFFICIENCY from January 2013
• Mazda CX-5 SkyActive-D from 2012
• Vauxhall Zafira 1.6 CDTi 136ps from August 2013
• Volvo V40 D4 190hp D4 from 2014
• Volkswagen Golf GTD 2.0 TDI from August 2013
• BMW 120d BluePerformance from 2012
• Mercedes-Benz E-Class E 350 and E 300 BlueTec from April 2013