My 2011 Volkswagen Passat Estate has had water ingress problems for years. It was supposedly fixed whilst under warranty, but now the warranty has expired and the water leaks have returned.

My local Volkswagen dealer says the problem is my responsibility and I've been quoted £4000 to fix it. However, I can't prove the problem existed under warranty and neither can Volkswagen (they say it's a different ingress problem)- is there anything I can do?