diesel spill stain on black paint

Have you tried using something like petrol, or carburettor cleaner to shift it? This may make things worse of course...

I had massive (and longstanding) diesel stains on both my white 1998 Fiat car and my white 1993 Ducato van, both of which I managed to remove with some petrol, so it seems that the paint quality on an elderly Fiat is better than on a modern Audi? I don't think this is the kind of marketing message that Audi UK wants, so I'd suggest that talking to the dealer and then to Audi themselves if necessary is perhaps a good plan.

The paint quality on your Audi appears to have more in common with my Lotus Elise: I bought that brand new and after 2 years the paint started falling off the passenger door. In the opinion of several paint specialists it was a manufacturing fault, so I contacted Lotus who refused to co-operate and said it was my problem to fix at my cost. That was a £30,000 car and I'll never buy from Lotus again, nor recommend anybody else to!

You can see a couple of pictures of my Lotus paintwork at http://www.LotsOfTroubleUsuallySerious.com (created before we got into the website business I hasten to add!) and it's a lot worse now. I'd suggest you ask Audi if they feel their sales will be improved by obtaining a reputation for build quality down with Lotus', and worse than Fiat, or whether they'd like to accept that you have a fault that should be fixed at their cost?