I had the exact same problem as you .. I only got fumes in the car when it was stationary.
You checked to see if there is any black soot on the turbo manifold ??
I had my turbo replaced when my car was only 8 months old. Faulty batch number apparently. When they replaced the turbo with the new one they damaged the flanged face on the turbo which left a small gap that the fumes could escape from.
Arnold Clark tried repairing the damage using some sort of red mastic. I went mental with them and told them I would not take the car until the turbo was replaced. Chancers !! .... They replaced the turbo and all is well now..