Road Test: Mercedes-Benz CLS CLS 250 CDI 4dr Tip Auto

Like Prince William marrying into the masses, the CLS has gone mainstream. In the first-generation car, you could only get a six- or eight-cylinder engine, but now there's a four-cylinder available. Merc is hoping this 2.1-litre diesel won't dilute the upmarket image of the CLS, but will boost sales by making it more attractive to company-car buyers.

So, the CO2 figures drop from 160g/km (CLS 350 CDI) to 135g/km for this CLS 250 CDI, while fuel economy is improved, from 46.3mpg to 54.3mpg. The 250 CDI is also £3,368 cheaper - not as much as you might expect maybe, but useful.

There is other good news with this lesser-CLS, as power (201bhp) and torque (369lb ft) are still good. Sure, it doesn't have the 350 CDI's effortlessness, but there's enough punch that you never have an anxious overtaking moment. Munching motorways is also painless, as there's plenty of poke from 60mph upwards, and that peak torque figure kicks in from a usefully low 1,600rpm. Merc's smooth seven-speed auto has also been re-engineered to work better with the four-cylinder engine, so the 250 CDI is suitably undemanding.

That's the good news, then. The bad news is that you shouldn't buy it. Or if you do, don't ever sit in a 350 CDI, as it'll show just how smooth Merc diesels can be. It purrs away virtually silent in the background, whereas this four-cylinder unit grumbles constantly. It sounds rougher than the V6, most notably when cold and at higher revs.

To be fair, this criticism needs quantifying. By most standards, the Mercedes four-cylinder is OK, but the boys from Stuttgart can, and should, do better. As standard, the CLS 250 comes with comfort suspension, which should be perfect for this GT car. But the 250 is too squishy, and there's an odd frequency thing going on at higher speeds - you move up and down in the seat at a different rate to how the car moves up and down. The seats' springs and the car's springs don't match.

So - for what must be the only time in TopGear history - we'd recommend getting a German car on sport suspension. The low-speed ride is a bit more fidgety, but the pay-off at higher speeds is more than worth it. It feels a lot more planted. But even with this fitted, we still wouldn't have the 250 CDI. As an overall package, the engine refinement isn't good enough. The CLS looks, and is priced, exclusively. But if the CLS is meant to be upmarket, it needs to feel it.