If Seat’s default service regime is like VW’s, then new cars from the factory are all set for long life (flexible) servicing, which is
up to a maximum of 18,000 miles or two years between services before the service indicator in the car prompts for the car to be serviced. The product code on the vehicle data
sticker for long life servicing is Q16 (I think this is the same for Seat and VW).
As Scroggs has said, long life servicing is condition based. The long life (flexible) service regime doesn’t necessarily mean you’ll be able to go 18,000 or two years before the car needs an oil change service; 18k miles or two years is the maximum. I believe the car monitors the engine oil quality / condition and if the type of motoring you’ve done results in the oil quality deteriorating to the point that it an oil change service is required before 18k miles / two years, then it will trigger a ‘service due’ warning. Sounds as if this is what has happened in your situation OP.