WOW thank you all for the warm welcome!
The car idles perfectly fine, no juddering or fluctuation of revs when he car is standing still. The vibration only happens when I'm creeping forward in traffic without any gas being applied. This problem is intermittent, only happens sometimes. I think I've noticed a pattern, it seems to happen when the car is warm (ie come of the motorway). The vibration is very noticeable, but only happens when you creep forward - Hope that makes sense!
I also notice sometimes when coming to a stop, when the car downshifts to 1st/2nd, the downshift feels rough/harsh causing the car to leap a bit (engine brake)? Is this normal?
I've read through a few threads about gearbox oil changes etc. I've read that if the car has exceeded 60k miles (mine is currently on 84k miles) it is not recommended to change the oil as it may result in harming the gearbox rather than doing it good as the oil has debris in there which helps protect worn components within the gearbox.
With regards to the MPG, I drive in rush hour traffic through the city for 10 miles and 10 miles on the motorway. I average 22mpg. On the same journey at the same time in my Nissan 350z I was getting a return of 23-24mpg. Considering the 6 is a convertible (heavier), bigger engine and an automatic; can't complain with the return!
According to my service history record book (my current mileage is 84k), the spark plugs were changed at 64k on 13/08/2009. The iDrive is showing the spark plugs have 44k miles remaining and it has been driven for 20k miles/3 years since they were changed. Does that tally up? How often do spark plugs need to be changed? These spark plugs were replaced at a Mercedes Specialist and not at BMW - would you think there is a possibility they replaced the spark plugs with the wrong ones?
Everything on my iDrive is showing 'OK'. Can the seller 'fiddle' with it to show 'OK' when it really isn't?
Photos are below: