Have to say my Rover 45 diesel, is still giving me astounding MPG, even at 16 years old and 185000 miles. It's now got a Pierburgh MAF and a MAF amplifier fitted, which means it's slightly more nippy than the standard setup, but even now, I'm getting 40+ mpg as long as I'm not ridiculously heavy footed.
I've not had any experience with the BMW lump in the 75, but the L series I'd recommend to anyone. Look after it and it'll look after you.