Seem to remember summit about M.B. NOT using DPF's for some reason or other on any of the cars they build for the UK....
Also, they wont use the new refrigerant in their new cars either--summit to do with explosions and safety, I dunno why, the new refrigerant is Propane, Cant think why they wouldnt use it!
Could try Fuller Fuels in Widness if thats anywhere near you.
Although as others have said, If you can afford a new M.B. and only using 25L per week,--then you can afford to Run the thing--Money should be little object!