In December's Top Gear magazine, I really liked the engine in the 135i Coupe. Almost made it sound like it was the engine to have in the range. Well, now that needs a teensy bit of alteration.

Because the one to have (and this is definitive because there won't be any more variants, or at least I hope there won't) is thae 2.0-litre twin-turbo diesel in the 123d.

The raw figures are seriously impressive. 0-62mph is seen off in 7.0 seconds, which is simply staggering for a car that returns 54.3mpg and sneaks under 140g/km of CO2. And all this in a car that weighs the best part of 1,500kg. That's an awful lot of metal to lug around.

It's not all about power though. On the launch, all the cars were already warmed up by the time I got a go, but BMW's diesels are more refined than they used to be and there's no reason to suspect this would be any different. It's certainly smooth when you rev it.

This is all courtesy of the same sort of principle as the 635d on the page opposite, in that a large turbo and small turbo run in tandem to provide instantaneous power whenever it's needed. And, boy, is it instant.

It's startling how quickly this thing reacts to your inputs so that you can heel and toe like in a petrol, and how sharply the rev needle zips round the dial. Crack on in this car and you'll be changing gear as often as in a hardened sports car.

The ride will make you feel like you're driving one as well. Do not, under any circumstances, buy the 123d in the M-Sport trim. The ride is far too hard, not at all polished and quite frankly unforgivable. You might as well sling it on four roller skates for all the cushioning these tyres and springs give. Which is such a shame, because the 123d has the engine to be as flexible as your mood takes you.

If you're relaxed or want a thrash, this motor lets you be as wild or calm as you like.

Piers Ward