advice.
take it to another garage, even if its a dealer. half of these garages dont even know how a gearbox works, let alone put one back together properly. Or ring SEAT UK, Google their number and ask to speak to a supervisor in Customer Services who can make decisions.
If you still arent getting anywhere, go and seek a solicitor, you might have to pay for an hour, but it will be worth it and youll get a decent letter out of them to send to the garage , making sure you copy in Seat UK ofcourse.