That’s ridiculous! Was it the dealer or Seat Customer Services who told you the drivers seat isn’t covered under warranty?
If the Seat new car warranty doesn’t specifically state that the drivers seat isn’t covered under the terms of the warranty, then it’s covered. If you don’t have a copy of, or access to, Seat’s new car warranty, I’d be asking for a copy of the warranty terms and conditions that shows the driver’s seat is excluded. I’d be very surprised if it had such an exclusion.
In the VW new car warranty, there is a wear and tear restriction, because VW deem that any wear to the seat facings can be influenced by the seat occupants. Having said that, they are covered for 6 months/6,500 miles, and some forum members on VW forums have had seat covers replaced outside of the 6 month/6,500 restriction.
I would expect Seat’s new car warranty to be no more restrictive than VW’s, and I wouldn’t accept what you’ve been told.
Irrespective of what you’ve been told, there’s general consumer legislation (Consumer Rights Act?) that I’m sure would state that goods have to be fit for use and of merchantable quality for a period much longer than 3 months.