Interstar diesel running rich

Sounds nasty! I had a similar problem with my old BMW 3 series, also diesel, a while back. Check engine light on, rough running, and the smell of unburnt fuel was intense. It turned out to be a faulty injector. Have you checked for any obvious leaks around the injectors or the fuel lines? Also, it might be worth getting the error codes read – that'll give you a much better idea of where the problem lies. Has the catalytic converter been checked?

That's helpful to know it could be the injectors. No obvious leaks that I can see. Where exactly should I be looking around the injector? What do you mean by "error codes"?

By error codes, I mean the fault codes stored in the car's computer. You need an OBD2 scanner to read them; many garages will do this for a small fee. This will pinpoint the issue. As for the injectors, look for any dampness or fuel residue right where they connect to the fuel lines and the engine. Given all the symptoms, it really does sound like an injector issue causing the rich mixture. I'd strongly suggest getting it checked out by a workshop; playing around with the fuel system without proper knowledge can be risky.

Thanks a million for your help! I took your advice and got the error codes read; confirmed it was a faulty injector, just like you suspected. Had all four replaced in the end for 515€. Runs like a dream now!