I'm guessing about £150 but you should probably ask a fiat dealer for the cost. You don't have to take it to a dealer but the garage must follow the FIAT service information to the letter if you want to keep your warranty intact.
Your warranty will be invalidated. I year interval or, at or before 18,000 miles if that occurs sooner. There are other threads & comments posted by the forum Moderator and Fiat are very clear about what they regard as "service intervals" and the tolerances (mileage & time) are quite tight.