Hello Karen,
Sorry to hear you have water ingress problems with your brand new car.
To me, the situation is not at all complex, and you don't have to worry about the reason why, and simply have to return it to the supplying dealer and request they correct the defect under warranty. Any new car which leaks water is surely unfit for purpose, and if they cannot effect a permanent cure then you are within your rights to reject the car. Although not mechanical, this would be an unacceptable problem to me since the boot will stay damp, will smell and lead to deterioration of the structure of the car - just not on with a brand new car. Ignore comments about the weather being exceptional bad! No excuse.
It might be worth doing a bit of research on this and I think you will find that this is not an unknown/uncommon problem with these cars, and Audi should know how to fix it by now. Some degree of continued pressure may be necessary to ensure it is fixed properly.
Goid luck with it.
Gareth.