It's covered under warranty but the end piece which is the bit that will need replacing is only about £30. If you have the typical £100 excess then you're better off doing it yourself.
I actually got mine done under warranty but only because it was done under a bigger warranty claim. They also fit some plastic spacers to the metal rod the bottom of the seatbelt attaches to, which helps the seatbelt assistant pick up the belt more reliably.
Excellent, thanks!