Road Test: BMW 5 Series 530d SE 4dr

Stonking engine, this. Fast, smooth, refined and quiet, with just a hint of six-cylinder diesel growl that helps make this 530d one of the finest engines in any car.

Unfortunately, you probably shouldn't buy one. If you've got avault of cash then go ahead. Buy away. But for nine grand less, you can get the equally magnificent 520d.

The cash-chasm is mainly accounted for by the engine. It also buys you auto aircon, metallic paint, a USB connection and ambient interior light, but the rest of the difference is swallowed by an extra two cylinders, 61bhp and 118lb ft of torque.

And this is where it gets tricky. As the financial questions niggle away in the corner of your mind, the 530d blasts them away with great swathes of torque. It's like it propels itself into some higher dimension, delivering pace and grace with almost complete disdain. It's also deceptively rapid, dispatching 62mph in 6.3 seconds.

But, as you can read here, the 520d handles itself similarly splendidly. It might not have quite the magic as its big cousin and, yes, it's slower... but nine grand slower? Hmmmm. We wonder.

Taken in isolation, the £37,100 cost doesn't seem so bad for such a well-sorted machine. But unless the engineers somehow crap-ify the 520, it's hard to justify the extra spend. Looks like BMW has become a victim of its own success. Bümmer.