Consumer law states that a product has to work for a reasonable amount of time. If a product fails before 6 months it is deemed that the defect was there at the time of purchase and as such it's responsibility of the seller to prove otherwise. After 6 months it's the other way around.
Definitely have a Google of consumer law for more details.
It could be the rear window seal, boot seals.
Here's a fellow member who had the same issue: