Hello Elaine,
Thanks for being in touch with the forum, and sorry to hear of your issue.
Apologies that this is what it is, and is pessimistic, but regrettably, I think it’s highly unlikely that anyone on here would be able to pinpoint the fundamental problem with the vehicle (with no access to it) when the combined technical experience at Audi (dealership?) and a diesel specialist have failed.
Sorry, I don’t want that to be dismissive, but constructive, and if it were mine, I would be concentrating my mind on taking legal advice (CAB) to reject the vehicle as unfit for purpose - given that you have allowed the seller plenty of opportunity to rectify the problem - and he has failed (not for the want of trying!)
I’m not sure what the consumer law is in NI, but there will be a time limit on the rejection, so there might be some urgency to gather the information you would need to go forward along this route.
I really don’t see it’s right that you should be worrying yourself about its cause. Its final resolution? Yes.
Perhaps you could let the forum know how you get on.
Kind regards,
Gareth.