So things have been quiet on this one for a while... This is quite a long post summing up my experience over the last few months.
I ran an air dam for about 8 months with no significant gains. It was pretty low as you can see on the grass:
The MPGuino is dialled in and pretty consistent with my "pump maths" readings.
Without the air dam I'm seeing UK MPG varying between 25 (town) and 40+ (straignt roads in warm weather not exceeding about 40mph).
In winter I'll be lucky if I top 30-35mpg in general use. To decrease warm-up time I tried a partial grille block on an aftemarket grille. That caused a hesitant engine on light throttle. I'm guessing that is probably because it made airflow turbulent at the intake, which is situated behind the grille (but in front of the rad).
My next project will prpbably be looking at an undertray, however, I can see from Hutcho that Mercedes put a fair bit of tunnel time in on this car.
The area I think can be improved is underneat the back end - there is a bumpercover and exhaust hanging further down than anything else. It's a relativeley messy area. I will photograph the underbody next time I get it up in the air.
The underbody is reasonably flat and flow is helped by flat covers under the engine, and fuel pump area.