The filter must be changed and it will still smell faintly for a while as the soot and smell will be impregnated all along the air vent tracks and in the seats etc.. I fabreezed all my seats, carpets, etc etc to
help get rid of the smell once Seat had done the fix.
If it isn't leaking from the joint between the turbo and the dpf then it must be
lower down post cat but that would be easier to see on a ramp. With mine when on a ramp with a torch you could look up at the turbo to dpf joint and see that there was soot around there, there was also black staining under the bonnet and under the sound deadening.
The smell is because the fresh air intake is in the middle of the scuttle trim, right above this joint. There is a rubber seal at the back of the bonnet and it keeps most of the engine bay fhimes out normally but if that joint is leaking it won't keep them all out.
Having tried to change the gasket my self it is a pig of a job unless you have the right tool to close up the clamp , and getting the gasket to sit right was also a pita, so that's why I sacked it back to seat.
Also depending how old yours is there is a known issue with the dpf mounting bracket which needs to be modified to allow a little more wiggle room when aligning everything, Seat have an official TSB for this mod which again you can ask them to do (apparently my 62 plate is late enough that it already has it, and I really struggled to get mine to line up, so god only knows how bad earlier cars are as naturally mine would not pull up straight).