Thanks for your reply
The apps on a smart TV only work during the period that the manufacturer of the set pays the licensing fees for those apps. (Hundreds of thousands of people have been enraged to find out that certain apps suddenly stop working on their TV sets, only two or three years after they've purchased them, because the manufacturer has withdrawn support for the app versions that are on their sets). That's why many (most?) technology journals advise against buying smart TVs and instead using add-on boxes (such as those offered by NOW TV) where the service providers need to keep supporting their apps in order to for people to trust them.
Either:
(a) you've bought a really new model offered by Samsung (and they've yet to commence paying the licensing fee for the iPlayer app) ; or
(b) you've bought a discontinued model and Samsung have already stopped paying the licensing fee.
A possibly relevant link:
http://conversation.which.co.uk/technology/smart-tv-software-support-commitment-issues-bbc-iplayer-netflix/
Contact Samsung to ask them what's going on:
http://www.samsung.com/uk/info/contactus.html