I've wondered that myself, I have 335d crossing the atlantic right now and I looked into this prior to ordering. I could not find any instances where the engine (M57TU2D30) suffers from intake coking. I've read that the "EGR" system is heated which I'm guessing was done to help prevent coking. IIRC there was a coking problem on the engine used for the 320d and possibly previous variants of the M57 (M57 block has been around since 1998). I know some older generations of the VW TDI sufferred from coking, but the newer model ('09 Jetta/Sport Wagon to present) is not supposed to have it either. I've read that volvo has some problems as well. IMO it's the cars which spend most of their life in city traffic that have developed the problem.
The UK forum is a good place to ask.