NTL rent you an STB as part of the package. Sky give you a STB (and dish) as part of the sign-up deal. That means the box belongs to you, which is why you might want to insure it for repair or replacement after the first 12 months.
However, some things to consider:
1. The company that phoned you is not Sky. It is an independent insurer that buys customer details from Sky so it knows who to target. It will do its damndest when contacting you to make you believe it
is Sky without actually telling you an outright lie.
2. £80 is steep and you may well be able to make any repairs/replace parts yourself for less, especially if using eBay. It is not unheard of for Sky to replace out-of-warranty boxes anyway, so as not to lose your custom.
3. Some fates that may befall your box and that are covered by this insurance will be covered by your home contents policy anyway.
When they phoned me, I told them where to go ...

Politely of course.
---------- Post added at 16:33 ---------- Previous post was at 16:32 ----------
Moved into a new thread.