Thanks for all the comments, managed to do a few more miles this week. Seems I have the usual "pull to the left" problem so it's in with the dealer on Friday although not expecting them to fix it. Will be off to wheels in motion if not and then merc will get the bill.
Andrew, you had a similar problem with you c63 didn't you? Did wim fix it for you?
As for the Diesel engines I am more talking about the 4 cylinder units. The merc is quite rough and the BMW is better but can still sound like a proper diesel at times. Our tt sounded much better and not as rough but this my opinion :-)
I am only just averaging 30mpg and 27 into work this morning :-0
MG/SS M4 ZCP