The big changes under the bonnet are the product of BMW's Efficient Dynamics philosophy, which simultaneously aims to reduce CO2 emissions and improve economy while boosting performance.
To this end, engine stop-start is standard not only with the six-speed manual gearbox, but also the new eight-speed automatic transmission.
The extensively revised 116i and 118i petrol engines are substantially faster and more economical, while scoring big reductions in CO2 emissions.
The 116i's power output rises by 15bhp to 136bhp, its 0-62mph time drops to a brisk 8.5 seconds, yet its fuel consumption improves to 49.6mpg and CO2 emissions fall 14g/km to 129g/km.
It's a similarly impressive story for the 118i, whose 170bhp output has risen from 141bhp for a distinctly sporty 7.4 second sprint to 62mph, yet it has slightly better fuel consumption and emits 9g/km less CO2 at 134g/km.
The gains made by the diesel 116d, 118d and 120d are fairly slight, on the other hand, although the numbers remain competitive the 116d makes 62mph in 10.3 seconds and returns 62.8mpg.
Canny buyers will look for the new 116d Efficient Dynamics model, whose 116bhp 1.6-litre engine produces just 99g/km of CO2 and is capable of 74.3mpg, which is a real step forward.
The Efficient Dynamics arrives in March, a three-door comes in a year and more powerful petrols and diesels after that.