I had a not dissimilar experience with a used VW Passat 2.0 Tdi.
From day one it would burn a lot of oil but initial visits back to the garage (indy) would just result in nonsense excuses and claims that some do burn a lot. I was getting through a litre in as little as 300 miles so clearly something was up.
I managed to contact the previous owner who, it turned out, had exactly the same problem but returned it to a different dealer. After this it was auctioned off and unfortunately ended up on a forecourt near me.
My dealer was less than impressed at my discovery but refused my initial demand of a refund claiming (rightly) that they had an obligation to repair it. In the end it needed a new engine as the rings were burnt out. Petrol in the tank being the most likely cause.
After the new engine the car was transformed but every year for about 3 years afterwards something big always went wrong.
I really wish I had pushed to get the refund.