Smell through vents

[Billjag]

Hello, I own a 2006 Stype Jaguar 2.7 92364 miles, diesel SE auto. There is a sooty smell coming in through the vents, it was in a garage they did a smoke test, and changed a part just in the engine bay, behind the dash, can't remember what the part is called, but the part was £680 plus fitting. The problem is now different before the snell was burny, now it is sooty, does anyone know what the problem could be