The new BMW 1-series coupé is a hugely significant car. Forget the paltry projected sales figures (BMW GB expects to sell no more than 3600 a year of all variants), and disregard the, er, challenging visual detailing. In fact, you can even ignore the awesomely powerful 135i that we drove last week.

Why? Because I reckon history will record the 1-series coupé as important for one reason, which is that it marks the debut of BMW’s brilliant 201bhp twin-turbo four-cylinder diesel.

Consider a few key facts. The 123d is the most powerful four-pot turbodiesel you can buy. It’s also one of the greenest. So you get 295lb ft of torque, 0-62mph in 7.0sec and a 148mph top speed at the same time as 54.7mpg and 138g/km of CO2 emissions. That’s the same CO2 output as a Skoda Roomster diesel.

The 123d’s performance on the road is equally impressive, with a smooth, linear power delivery providing decent shove from 1800rpm right up to 4500rpm. It’s pretty refined, too, though not as sonorous as BMW or Audi six-cylinder diesels.

For the rest of the car, the praise is a little less gushing. As with the 135i, the 123d is a neat and tidy handler, with an ultimate tendency to understeer. But unlike the 135i, the steering is electrically assisted and, while accurate, seems oddly detached, almost like an arcade game with fake force feedback.

The interior is impressive enough, with plenty of space and great-quality materials, particularly the Boston Red leather of our test car (a £965 option). Which brings us to the one major weakness of the 123d: price. In M Sport guise you’ll be forking out £26,290 before options. That’s at least three grand more than the equivalent Audi A3.

But an over-the-odds sticker price isn’t enough to spoil the 123d; this is still a distinctive, interesting car. With a brilliant engine.
MATT RIGBY