Mini Cooper Diesel Running Rough

A rich mixture on a diesel, especially with those symptoms, does point to injector issues. When was the last time you changed your spark plugs and air filter? Has the check engine light been on consistently, or does it come and go? Any recent work on the intake manifold?

Why the intake manifold?

The intake manifold question relates to potential air leaks, although with a constant check engine light and your symptoms, injector failure is the more probable cause. A diagnostic check would confirm which injector is faulty. Given the mileage, it might be wise to replace them all. I recommend taking it to a qualified workshop.

Turns out, injector number three was toast. Replaced all four, and she's running like a champ again. Cost me 515€ but worth it.