For what it's worth, the process is exacly the same on the 320D, although the filter is a little shorter. I also paid under £20 for a MANN filter, but from GSF. I've only had my car a few weeks and decided to do all the filters, even the fuel filter under the car. The Air Filter took me around 1hr 30mins including the cursory half time tea break.
I reckon I'll do it in under 1 hour next time
I havn't noticed any diference in mpg, but I've averaged 46.3mpg over 3,000 miles