I also had a bitching Honda dealer similar to you reading me the riot act, he even said that the car was badly built as it would have been desinged for the Cypriot Hire Car market, so built with poor quality parts!! Unbelivable!!
Anyway, WRT to the warranty, unless you can prove (stamped and verified by VAT registered dealer ) it's been serviced at 1 yr old/10k miles your warranty is invalid (even the 2 year manuafacturer one), the UK cars come with that plus 1 year dealer extension. MotorPoint don't have to do anything as it wouldn't have needed a service at that point (when you took ownership), so it's your responsability to make sure it has been serviced to Seats requirements since then, which from what you've said you haven't......
Lesson to be learned, do your homework and check before leaving that you've got everything as agreed before driving the car off!!
Thinking back, I should have expected no less from the Dealer with regard to import status. However, the V5 looks like it's a UK Car so I should get 3 year warranty.
Only problem i've got left now is proving the service(s), if any.
If SEAT don't hold any information, i'm screwed.