C Class Mystery Fault

Hopefully somebody can assist. I have late 2019 C300d 4matic estate. At random times and for no apparent reason it suddenly massively increases fuel consumption and when I stop the car there is a smell of burning friction material from the rear of the car. My local Mercedes garage keeps telling me duff information, presumably because they have no idea. The latest is that it is the knock sensors causing the increase in fuel. Changing from 50mpg to 20mpg is not that..... Nor is it the DPF regenerating, I only do long runs in the car, usually 200+ miles at a time. DPF regen does not smell like burning friction material either. There first attempt at a fix was to reflash the ECU.

I know of one other C Class diesel that had this problem back in 2015 or so. That was a new car and was rejected back to the supplying dealer. According to the owner it is a known fault that Mercedes are keeping quiet.

Any ideas?

Many thanks.

Dealer has agreed to take the car back, now to look for something else.