Is there a particular reason you are looking for the diesel model ?
I decided to buy an S Class back in February this year. I was initially searching for an S350 ( most are LWB and that was what I wanted ) however I was particular in the fact that I did not want black exterior and I did not want a black interior.
My budget also meant I was looking at 2014/15.
I was not having much luck for a couple of weeks searching and was also wary that most black ones may have been "Knightsbridge Taxis ", it seemed the odds were against me.
Suddenly I thought about getting a petrol car as this would probably rule out the majority that may have been Chauffeur cars.
Bingo !! I found a 2015 model S500 from a Mercedes main dealer, it was one private owner and 40000 miles, full history and Mercedes warranty etc etc. The best thing was it was Diamond Silver with Crystal grey and seashell grey interior, it even had the rare Magic Body Control as well.
With careful driving I have had as much as 33mpg but I do tend to drive a bit more quickly and it is still upper 20's so very pleased about that.