I think I recall that my car (a 2011 model CRDi auto) had service interval of 12,500 for 1st year then 20,000 miles or 1 year thereafter, but cant be sure. I also recall that some of the early 2011 models (like mine) has wrong initial interval published in the service book supplied.
If you want corroboration of what dealer has said then why not email Hyundai UK customer service? I think email address is in one of the booklets (warranty maybe?).
Notwithstanding that, some would advocate changing the engine oil more frequently than @ 20,000 miles.
Ed